Duolingo (DUOL) 10-Q 12 Aug 21 2021 Q2 Quarterly report Financial data

Cover Page

Cover Page - shares	6 Months Ended
	Jun. 30, 2021	Aug. 11, 2021
Document Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2021
Document Transition Report	false
Entity File Number	001-40653
Entity Registrant Name	Duolingo, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	45-3055872
Entity Address, Address Line One	5900 Penn Avenue
Entity Address, City or Town	Pittsburgh
Entity Address, State or Province	PA
Entity Address, Postal Zip Code	15206
City Area Code	412
Local Phone Number	567-6602
Title of 12(b) Security	Class A common stock, $0.0001 per share
Trading Symbol	DUOL
Security Exchange Name	NASDAQ
Entity Current Reporting Status	No
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Shell Company	false
Entity Ex Transition Period	false
Entity Central Index Key	0001562088
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q2
Amendment Flag	false
Common Class A
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		12,819,870
Common Class B
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		24,598,497

UNAUDITED CONDENSED CONSOLIDATE

UNAUDITED CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 114,636	$ 120,490
Accounts receivable	25,129	20,450
Deferred cost of revenues	17,168	13,585
Prepaid expenses and other current assets	5,635	3,855
Total current assets	162,568	158,380
Property and equipment, net	7,692	6,428
Capitalized software, net	3,731	2,296
Operating lease right-of-use assets	6,805	8,073
Other assets	897	562
Total assets	181,693	175,739
Current liabilities
Accounts payable	4,266	2,196
Deferred revenues	70,997	54,792
Income tax payable	9	68
Accrued expenses and other current liabilities	9,779	8,634
Total current liabilities	85,051	65,690
Long term obligation under operating leases	7,810	8,131
Total liabilities	92,861	73,821
Commitments and contingencies (Note 9)
Convertible preferred stock, $0.0001 par value, 19,074 shares issued and outstanding at December 31, 2020 and June 30, 2021	182,609	182,609
Stockholders’ equity:
Common stock, $0.0001 par value; 42,800 authorized shares; 13,271 issued and outstanding at June 30, 2021 and 12,794 issued and outstanding at December 31, 2020.	1	1
Additional paid-in capital	30,649	30,087
Accumulated deficit	(124,427)	(110,779)
Total stockholders’ deficit	(93,777)	(80,691)
Total liabilities, convertible preferred stock and stockholders' deficit	$ 181,693	$ 175,739

UNAUDITED CONDENSED CONSOLIDA_2

UNAUDITED CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares shares in Thousands	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020
Statement of Financial Position [Abstract]
Convertible preferred stock, par value (in usd per share)	$ 0.0001	$ 0.0001
Convertible preferred stock, shares issued (in shares)	19,074	19,074
Convertible preferred stock, shares outstanding (in shares)	19,074	19,074	18,488
Common stock, par value (in usd per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares)	42,800	42,800
Common stock, shares issued (in shares)	13,271	12,794
Common stock, shares outstanding (in shares)		12,794	13,271

UNAUDITED CONDENSED CONSOLIDA_3

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Income Statement [Abstract]
Revenues	$ 58,803	$ 40,011	$ 114,163	$ 68,123
Cost of revenues	16,137	11,809	31,156	20,023
Gross profit	42,666	28,202	83,007	48,100
Operating expenses:
Research and development	21,940	12,111	44,469	21,687
Sales and marketing	9,619	8,625	29,392	14,136
General and administrative	11,585	7,384	23,038	14,650
Total operating expenses	43,144	28,120	96,899	50,473
Loss from operations	(478)	82	(13,892)	(2,373)
Other income (expense), net	303	(31)	262	202
(Loss) income before provision for income taxes	(175)	51	(13,630)	(2,171)
Provision for income taxes	1	11	18	22
Net comprehensive (loss) income	(176)	40	(13,648)	(2,193)
Net (loss) income	$ (176)	$ 40	$ (13,648)	$ (2,193)
Basic loss per common share (in usd per share)	$ (0.01)	$ 0	$ (1.05)	$ (0.18)
Diluted loss per common share (in usd per share)	$ (0.01)	$ 0	$ (1.05)	$ (0.18)

UNADUDITED CONSOLIDATED STATEME

UNADUDITED CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' DEFECIT - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Increase (Decrease) in Temporary Equity [Roll Forward]
Beginning balance (in shares)	19,074	18,247	19,074	18,247
Beginning balance	$ 182,609	$ 137,686	$ 182,609	$ 137,686
Issuance of convertible preferred stock (in shares)		241		241
Issuance of convertible preferred stock		$ 9,976		$ 9,976
Ending balance	$ 182,609	$ 147,662	$ 182,609	$ 147,662
Ending balance (in shares)	19,074	18,488	19,074	18,488
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Beginning balance	$ (97,785)	$ (84,773)	$ (80,691)	$ (83,976)
Stock-based compensation	2,907	1,681	$ 5,458	2,835
Stock options exercised (in shares)			500
Stock options exercised	1,277	168	$ 3,307	450
Common stock repurchased and retired			(868)
Options repurchased			(7,335)
Net (loss) income	(176)	40	(13,648)	(2,193)
Ending balance	$ (93,777)	$ (82,884)	$ (93,777)	$ (82,884)
Common Stock
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Beginning balance (in shares)	13,118	12,456	12,794	12,406
Beginning balance	$ 1	$ 1	$ 1	$ 1
Stock options exercised (in shares)	153	26	500	76
Common stock repurchased and retired (in shares)			(23)
Ending balance (in shares)	13,271	12,482	13,271	12,482
Ending balance	$ 1	$ 1	$ 1	$ 1
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Beginning balance	26,465	12,462	30,087	11,026
Stock-based compensation	2,907	1,681	5,458	2,835
Stock options exercised	1,277	168	3,307	450
Common stock repurchased and retired			(868)
Options repurchased			(7,335)
Ending balance	30,649	14,311	30,649	14,311
Accumulated Deficit
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Beginning balance	(124,251)	(97,236)	(110,779)	(95,003)
Net (loss) income	(176)	40	(13,648)	(2,193)
Ending balance	$ (124,427)	$ (97,196)	$ (124,427)	$ (97,196)

UNADUDITED CONSOLIDATED STATE_2

UNADUDITED CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' DEFECIT (Parenthetical) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2020
Statement of Stockholders' Equity [Abstract]
Issuance costs	$ 24	$ 24

UNAUDITED CONDENSED CONSOLIDA_4

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Cash flows from operating activities:
Net loss	$ (13,648)	$ (2,193)
Adjustments to reconcile net loss to net cash used for operating activities:
Depreciation and amortization	1,236	1,022
Stock-based compensation	5,458	2,835
Changes in assets and liabilities
Deferred revenue	16,205	18,327
Accounts receivable	(4,679)	(7,460)
Deferred cost of revenue	(3,583)	(4,606)
Prepaid expenses and other current assets	847	(199)
Accounts payable	2,070	2,283
Accrued expenses and other current liabilities	(291)	2,412
Noncurrent assets and liabilities	612	759
Net cash provided by operating activities	4,227	13,180
Cash flows from investing activities:
Capitalized software	(1,656)	(123)
Purchase of property and equipment	(1,978)	(2,193)
Net cash used for investing activities	(3,634)	(2,316)
Cash flows from financing activities:
Net proceeds from issuance of convertible preferred stock	0	9,976
Proceeds from exercise of stock options	3,307	450
Repurchases of stock options	(7,335)	0
Repurchase of common stock	(868)	0
Payments of deferred offering costs	(1,551)	0
Net cash (used for) provided by financing activities	(6,447)	10,426
Net (decrease) increase in cash and cash equivalents	(5,854)	21,290
Cash and cash equivalents - Beginning of period	120,490	59,843
Cash and cash equivalents - End of period	114,636	81,133
Supplemental disclosure of cash flow information:
Cash paid for interest	0	0
Cash paid for income taxes	58	0
Supplemental disclosure of noncash investing activities:
Capitalized software included in accrued expenses	75	0
Property and equipment included in accrued expenses	226	0
Supplemental disclosure of noncash financing activities:
Deferred offering costs included in accrued expenses	$ 1,076	$ 0

DESCRIPTION OF THE BUSINESS AND

DESCRIPTION OF THE BUSINESS AND BASIS OF PRESENTATION	6 Months Ended
	Jun. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
DESCRIPTION OF THE BUSINESS AND BASIS OF PRESENTATION	DESCRIPTION OF THE BUSINESS AND BASIS OF PRESENTATION Duolingo, Inc. (the “Company” or “Duolingo”) was formed on August 18, 2011 and the Duolingo app was launched to the general public on June 19, 2012. The Company’s headquarters are located in Pittsburgh, Pennsylvania. Duolingo is a US-based language-learning website and mobile app, as well as a digital language proficiency assessment exam. The Company has a freemium business model: the app and the website are accessible free of charge, although Duolingo also offers a premium service, Duolingo Plus, for a subscription fee. As of the date of this filing, Duolingo offers courses in 40 different languages, including Spanish, English, French, German, Italian, Portuguese, Japanese and Chinese. We have locations in the United States and China. Principles of Consolidation —The unaudited condensed consolidated financial statements include the accounts of the Company and subsidiaries over which the Company has control. All intercompany transactions and balances have been eliminated. Basis of Presentation —The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States (“GAAP”) from the Company’s accounting records and reflect the consolidated financial position and results of operations for the six months ended June 30, 2021 and 2020. Unless otherwise specified, all dollar amounts are referred to in thousands. The unaudited condensed consolidated financial statements have been prepared pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”). Certain information and note disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such SEC rules. We believe that the disclosures made are adequate to make the information presented not misleading. In our opinion, all adjustments considered necessary for a fair presentation of the financial statements have been included, and all adjustments are of a normal and recurring nature. We consistently applied the accounting policies consistent with the annual consolidated financial statements elsewhere in this this Quarterly Report on Form 10-Q, in preparing these unaudited condensed consolidated financial statements, with the exception of accounting standard updates described in Note 2, “Recently Adopted Accounting Pronouncements.” These unaudited condensed consolidated financial statements should be read in conjunction with the audited financial statements and the notes for the fiscal year ended December 31, 2020 included in the final prospectus dated as of July 27, 2021 and filed with the SEC, pursuant to Rule 424(b)(4) on July 28, 2021 (“Final Prospectus”). Certain prior period amounts have been reclassified to conform to the current period presentation

SUMMARY OF SIGNIFICANT ACCOUNTI

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	6 Months Ended
	Jun. 30, 2021
Accounting Policies [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIESUnder the JOBS Act, emerging growth companies also can delay adopting new or revised accounting standards until such time as those standards would otherwise apply to private companies. While we have not historically delayed the adoption of new or revised accounting standards until such time as those standards would apply to private companies, we have elected to take advantage of this extended transition period and, as a result, our operating results and financial statements in the future may not be comparable to the operating results and financial statements of companies who have adopted the new or revised accounting standards. Accounting Principles —The financial statements and accompanying notes are prepared in accordance with generally accepted accounting principles in the United States of America (GAAP). Use of Estimates— The preparation of unaudited condensed consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Significant estimates and assumptions reflected in the consolidated financial statements include, but are not limited to, useful lives of property and equipment, valuation of deferred tax assets and liabilities, stock-based compensation, common stock valuation, operating lease right-of-use assets and liabilities, capitalization of internally developed software and associated useful lives and contingent liabilities. Actual results may differ materially from such estimates. Management believes that the estimates, and judgments upon which they rely, are reasonable based upon information available to them at the time that these estimates and judgments are made. To the extent that there are material differences between these estimates and actual results, the Company’s consolidated financial statements will be affected. Deferred Offering Costs— Deferred offering costs, which consist of direct incremental legal, accounting, and consulting fees relating to the Initial Public Offering (“IPO”), will be capitalized. The deferred offering costs will be offset against IPO proceeds upon the consummation of the IPO. In the event the planned IPO is terminated, the deferred offering costs will be expensed. There were no deferred offering costs recorded as of December 31, 2020. As of June 30, 2021, there was $2,627 of deferred offering costs recorded in the unaudited condensed consolidated balance sheet, of which $1,076 had not been paid. Cash and Cash Equivalents— Cash consists primarily of cash on hand and bank deposits. Cash equivalents consist primarily of money market accounts with maturities of three months or less at the date of acquisition and are stated at cost, which approximates fair value. The Company maintains cash deposits with financial institutions that may exceed federally insured limits at times. The following table shows the breakout between cash and money market funds. June 30, December 31, Cash $ 10,571 $ 20,428 Money market funds 104,065 100,062 Total $ 114,636 $ 120,490 The Money market funds are considered Level 1 financial assets. Level 1 financial assets use inputs that are unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date. Advertising Costs— Advertising costs were approximately $7,685 and $19,751 for the three and six months ended June 30, 2021 and $6,460 and $10,495 for the three and six months ended June 30, 2020, respectively, and are included within Sales and marketing in the unaudited condensed consolidated statement of operations and comprehensive loss. Income Taxes— The Company’s period provision for income taxes is computed by using an estimate of the annual effective tax rate, adjusted for discrete items taken into account in the relevant period, if any. Each quarter, the annual effective income tax rate is recomputed and if there are material changes in the estimate, a cumulative adjustment is made. Contributors— On March 10, 2021, the Company announced that it was ending its non-employee volunteer program, which began in 2013 to build and improve language courses. As part of this change, those contributors who participated in the program became eligible to receive a one-time award, up to an aggregate amount of approximately $5,098, including fees paid to process payments of approximately $526. The Company accounted for this under Financial Accounting Standards Board (“FASB”) Accounting Standards Codifications (“ASC”) 958-720, Not-For-Profit Entities - Other Expenses and ASC 720-25, Contributions Made , based on the nature of this contribution, which is an unconditional promise. The Company recorded the initial charge during the three months ended March 31, 2021, which is when the Company made the unconditional promise to pay. During the three months ended June 30, 2021 the amount previously reserved was reduced by $878 to reflect those awards that were not claimed, resulting in a reduction of the expense, bringing the total amount to $4,220. Of that amount, $577 is expected to be paid out in the three months ended September 30, 2021. This amount is included within Sales and marketing in the unaudited condensed consolidated statement of operations and comprehensive loss. Concentration of Credit Risk —The Company’s concentration of credit risk relates to financial institutions holding the Company’s cash and cash equivalents and platforms with significant accounts receivable balances and revenue transactions. The Company maintains cash deposits with financial institutions that may exceed federally insured limits at times. Management believes that the financial institutions that hold the Company’s deposits are financially credit worthy and, accordingly, minimal credit risk exists with respect to those balances. The majority of our revenue comes through our subscriptions and advertising streams and payments are made to Duolingo through service providers. The top two, Apple and Google, accounted for 66.4% and 18.3% of total accounts receivable as of June 30, 2021, respectively. The top three service providers, Apple, Google and Stripe, accounted for 47.8%, 28.9% and 13.8% of total accounts receivable as of December 31, 2020, respectively. Apple, Google, and Stripe, processed 51.6%, 29.3% and 10.3% and 50.9%, 28.8%, and 10.3% of total revenue for the three and six months ended June 30, 2021, respectively. Three service providers, Apple, Google, and Stripe, processed 50.0%, 26.5%, and 10.1% of total revenue for the three months ended June 30, 2020, respectively. Two services providers, Apple and Google, processed 52.1% and 27.6% for the six months ended June 30, 2020. Recently Adopted Accounting Pronouncements In August 2018, the FASB issued Accounting Standard Update (“ASU”) No. 2018-15, Intangibles-Goodwill and Other-Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That is a Service Contract (ASU 2018-15) . ASU 2018-15 aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. The Company adopted this guidance on January 1, 2021 and it did not have a material impact on its unaudited condensed consolidated financial statements and related disclosures upon adoption. Recently Issued Pronouncements Not Yet Adopted In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes

REVENUE

REVENUE	6 Months Ended
	Jun. 30, 2021
Revenue from Contract with Customer [Abstract]
REVENUE	REVENUE The Company has three predominant sources of revenue; time-based subscriptions, in-app advertising placement by third parties and the Duolingo English Test. Revenue is recognized upon transfer of control of promised products or services to users in an amount that reflects the consideration the Company expects to receive in exchange for those services. The Company does not enter into contracts with a customer that contain multiple promises that result in multiple performance obligations. Revenue is recorded net of taxes, assessed by a government authority that are both imposed on and concurrent with specific revenue transactions between us and our users. Revenue from time-based subscriptions includes a stand-ready obligation to provide hosting services that are consumed by the customer over the subscription period. Users can purchase Duolingo monthly or they can purchase a six-month or year-long subscription and pay for the subscription at the time of purchase. Therefore, such payments are initially recorded to deferred revenue. The user has the ability to download limited content offline. However, as there is a significant level of integration and interdependency with the online functionality, the Company considers the service to be a single performance obligation for the online and offline content. The Company enters into arrangements with advertising networks to monetize the in-app advertising inventory. Revenue from in-app advertising placement is recognized at a point in time when the advertisement is placed and is based upon the amount received. Duolingo English Test revenue is generally recognized once the tests have gone through the proctoring process and a certification decision has been made. This process usually takes less than 48 hours after the test has been completed and uploaded. Customers have 90 days from the date of purchase to take the exam or their purchase will expire and revenue will be recognized. Virtually all customers complete their exams prior to expiration. Sometimes organizations may purchase tests in bulk via coupons with a one year expiration date. The company will defer revenue from all tests that haven’t been proctored nor expired. The Company’s users have the option to purchase consumable in-app virtual goods. The Company recognizes revenue over the period in which the user consumes the virtual good, which is generally within a month. Principal Agent Considerations —The Company makes its application available to be downloaded through third-party digital distribution service providers. Users who purchase subscriptions also pay through the respective app stores. The Company evaluates the purchases via third-party payment processors to determine whether its revenues should be reported gross or net of fees retained by the payment processor. The Company is the principal in the transaction with the end user as a result of controlling, hosting, and integrating the delivery of the virtual items to the end user. The Company records revenue gross as a principal and records fees paid to third-party payment processors as Cost of revenues. Contract Balances —Deferred revenue mostly consists of payments we receive in advance of revenue recognition, and is mostly related to time-based subscriptions, which will be recognized into revenue over the course of the upcoming year, which is 12 months or less. Additionally, Duolingo English Test has deferred revenue related to tests that have been purchased, but will not be recognized until the tests have been proctored. Disaggregation of Revenue In accordance with ASC 606, Revenue from Contracts with Customers , the Company disaggregates revenue from contracts with customers into source of revenue, which most closely depicts how the nature, amount, timing and uncertainty of revenue and cash flows are affected by economic factors. Information regarding source of revenues: Three Months Ended June 30, Six Months Ended June 30, 2021 2020 2021 2020 Over time $ 43,502 $ 28,274 $ 83,557 $ 50,445 Point in time 15,301 11,737 30,606 17,678 Total revenue $ 58,803 $ 40,011 $ 114,163 $ 68,123 Information regarding revenue by stream: Three Months Ended June 30, Six Months Ended June 30, 2021 2020 2021 2020 Revenues: Subscription $ 43,502 $ 28,274 $ 83,557 $ 50,445 Advertising 9,056 6,808 18,331 11,816 Duolingo English Test 4,833 4,598 9,868 5,351 Other (1) 1,412 331 2,407 511 Total revenues $ 58,803 $ 40,011 $ 114,163 $ 68,123 ________________ (1) Other revenue is mainly comprised of in app purchases of virtual goods. Changes in deferred revenues were as follows: Six Months Ended June 30, 2021 2020 Beginning balance—January 1 $ 54,792 $ 26,307 Amount from beginning balance recognized into revenue (41,498) (19,833) Recognition of deferred revenue (45,378) (30,737) Deferral of revenue 103,081 68,897 Ending balance—June 30 $ 70,997 $ 44,634

PROPERTY and EQUIPMENT, net

PROPERTY and EQUIPMENT, net	6 Months Ended
	Jun. 30, 2021
Property, Plant and Equipment [Abstract]
PROPERTY and EQUIPMENT, net	PROPERTY and EQUIPMENT, net Property and equipment consists of the following as of June 30, 2021 and December 31, 2020: June 30, December 31, Leasehold improvements $ 9,336 $ 7,536 Furniture, fixtures and equipment 2,364 1,959 Total property and equipment 11,700 9,495 Less: accumulated depreciation (4,008) (3,067) Total property and equipment, net $ 7,692 $ 6,428 Depreciation expense was $488 and $940 for the three and six months ended June 30, 2021 and $378 and $720 for the three and six months ended June 30, 2020, respectively, and is predominately included within General and administrative, with nominal amounts in Cost of revenues, Research and development

CAPITALIZED SOFTWARE, net

CAPITALIZED SOFTWARE, net	6 Months Ended
	Jun. 30, 2021
Goodwill and Intangible Assets Disclosure [Abstract]
CAPITALIZED SOFTWARE, net	CAPITALIZED SOFTWARE, net Capitalized software consists of the following as of June 30, 2021 and December 31, 2020: June 30, December 31, Capitalized software $ 9,912 $ 8,181 Less: accumulated amortization (6,181) (5,885) Capitalized software, net $ 3,731 $ 2,296 Amortization expense of $148 and $296 for the three and six months ended June 30, 2021, and $243 and $302 for the three and six months ended June 30, 2020 is recorded in the Company’s consolidated statement of operations, respectively. Amortization expense is included within the following financial statement line items within the Company’s consolidated statement of operations: Three Months Ended June 30, Six Months Ended June 30, 2021 2020 2021 2020 Cost of revenue $ — $ 25 $ — $ 51 Sales and marketing 148 218 296 251 Total $ 148 $ 243 $ 296 $ 302 The Company reviews its long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. If the sum of the estimated undiscounted future cash flows expected to result from the use and eventual disposition of an asset is less than the carrying amount of the asset, an impairment loss is recognized. Measurement of an impairment loss is based on the fair value of the asset. No assets were impaired during the three and six months ended June 30, 2021 and 2020.

INCOME TAXES

INCOME TAXES	6 Months Ended
	Jun. 30, 2021
Income Tax Disclosure [Abstract]
INCOME TAXES	INCOME TAXES The income tax provision for interim periods is comprised of tax on ordinary income (loss) provided at the most recent projected annual effective tax rate (“PAETR”), adjusted for the tax effect of discrete items. Management estimates the PAETR each quarter based on the forecasted annual pretax income or (loss). The Company is required to reduce deferred tax assets by a valuation allowance if, based on all available evidence, it is considered more likely than not that some portion or all of the benefit of the deferred tax assets will not be realized in future periods. The Company also records the income tax impact of certain discrete, unusual or infrequently occurring items including changes in judgment about valuation allowances and effects of changes in tax laws or rates, in the interim period in which they occur. The actual year-to-date income tax expense (benefit) is the product of the most current PAETR and the actual year-to-date pretax income (loss) adjusted for any discrete tax items. Items unrelated to current period ordinary income or (loss) are recognized entirely in the period identified as a discrete item of tax. The income tax expense (benefit) for a particular quarter, except for the first quarter, is the difference between the year-to-date calculation of income tax expense (benefit) and the year-to-date calculation for the prior quarter. Items unrelated to current period ordinary income or (loss) are recognized entirely in the period identified as a discrete item of tax. The inclusion of discrete items in a particular quarter can cause the actual effective rate for that quarter to vary significantly from the PAETR. Therefore, the actual effective income tax rate for a particular quarter can vary significantly based upon the jurisdictional mix and timing of actual earnings compared to projected annual earnings, permanent items, earnings for those jurisdictions that maintain a valuation allowance, tax associated with jurisdictions excluded from the PAETR calculation and discrete items. Annual Effective Tax Rate The PAETR, which excludes the impact of discrete items, was (0.6)% and 21.6% as of the three months ended June 30, 2021 and 2020, respectively. The PAETR for the six months ended June 30, 2021 and 2020, was (0.1)% and (1.0)%, respectively. The PAETR was lower than the US federal statutory rate of 21.0% primarily due to the impact of maintaining a US valuation allowance provided on US deferred tax assets. The Company continues to maintain a full valuation allowance on U.S. federal and state net deferred tax assets (excluding the tax effects of deferred tax liabilities associated with indefinite lived intangibles) for the period ending June 30, 2021 as a result of pretax losses incurred since the Company’s inception in early 2012. The Company is projecting pre-tax loss in 2021. Current and Prior Period Tax Expense For the three and six months ended June 30, 2021, the Company recognized income tax expense of $1 and $18 on pretax loss of $175 and $13,630, respectively.

CONVERTIBLE PREFERRED STOCK

CONVERTIBLE PREFERRED STOCK	6 Months Ended
	Jun. 30, 2021
Temporary Equity Disclosure [Abstract]
CONVERTIBLE PREFERRED STOCK	CONVERTIBLE PREFERRED STOCK Convertible preferred stock is comprised of the following as of June 30, 2021 and December 31, 2020: Shares Series Authorized Outstanding Per share price Aggregate liquidation preference Funds received Fees incurred Carrying value of convertible preferred stock A 3,865 3,865 $ 0.85 $ 3,300 $ 3,300 $ 52 $ 3,248 B 6,298 6,298 2.38 15,000 15,000 60 14,940 C 2,948 2,948 6.78 20,000 20,000 112 19,888 D 3,154 3,154 14.27 45,000 45,000 146 44,853 E 1,224 1,224 20.43 25,000 25,000 92 24,909 F 758 758 39.57 30,000 30,000 153 29,848 G 241 241 41.38 10,000 10,000 24 9,976 H 586 586 59.77 35,000 35,000 52 34,947 Total 19,074 19,074 $ 183,300 $ 183,300 $ 691 $ 182,609 Each share of convertible preferred stock has a liquidation preference over common stock equal to the original issue price of the preferred stock plus any declared but unpaid dividends. No single class of preferred stock has liquidation preference that is senior in payment to other classes of preferred stock. The preferred stock does not accumulate undeclared and unpaid dividends. Each share of convertible preferred stock is convertible into shares of common stock at the option of the stockholder based upon a conversion rate that is 1:1 and may be adjusted under certain circumstances as defined in the Company’s Amended and Restated Certificate of Incorporation. If the Company consummates a public offering from which the Company receives gross proceeds of at least $50,000, the conversion becomes mandatory for the convertible preferred stockholders. Also, the conversion becomes mandatory for a preferred stock class, if the holders of at least 65% of the then outstanding shares of preferred stock elect to convert. The Company has reserved an equal number of shares of common stock for the potential conversion of each series of convertible preferred stock. The convertible preferred stockholders have voting rights equal to common stockholders. The preferred stockholders are entitled to the number of votes equal to the number of shares of common stock into which the preferred shares could be converted. All classes of convertible preferred stock are being classified as outside of stockholders’ deficit as the preferred stock has redemption features that are outside of the Company’s control upon certain triggering events, such as a “Deemed Liquidation Event.” In the case of a Deemed Liquidation Event, the holders of Preferred Stock are entitled to be paid out of the assets of the Company, prior and in preference to any distribution to the holders of the common stock. Because the Company’s common stockholders do not control the Company’s Board of Directors, a potential Deemed Liquidation Event is considered to be outside of the control of the Company, resulting in classification outside of stockholders’ deficit.

STOCK-BASED COMPENSATION

STOCK-BASED COMPENSATION	6 Months Ended
	Jun. 30, 2021
Equity [Abstract]
STOCK-BASED COMPENSATION	STOCK-BASED COMPENSATION The Company has a stock incentive plan whereby the Board of Directors may grant stock options or restricted stock units to employees, directors and consultants to purchase shares of the Company’s common stock under the 2011 Equity Incentive Plan (Plan). The Company has authorized 10,113 common shares to be issued under the Plan. The Plan permits the granting of incentive stock options and nonqualified stock options. The Company’s stock options vest based on terms in the stock option agreements and generally vest over four years and have a term of ten years subject to the continuous service to the Company by the optionee. Incentive stock options may be granted at an exercise price of not less than 100% of the estimated fair value of the stock at the date of the grant as determined by the Company’s Board of Directors. If incentive stock options are granted to a stockholder who owns more than 10% of the voting power of all classes of stock of the Company, the exercise price of the incentive stock options must be at least 110% of the estimated fair value of the common stock at the date of the grant. In November of 2020, the Board approved the amended and restated Plan, which provides for, among other things, the ability of the Company to grant restricted stock units (RSUs). Each RSU award vests based upon the satisfaction, during the term of the RSUs, of two requirements: length of service and a liquidity event defined as a change in control or a qualified IPO. No compensation expense has been included for RSUs as the vesting conditions are not probable. In June of 2021, the Company granted 1,800 performance-based RSUs to the Company’s founders (the Founder Awards). The Founder Awards vest upon the satisfaction of both a service-based condition and a performance-based condition and generally are settled 1 year after vesting. The service-based condition is satisfied as to 25% of the Founder Awards on each anniversary of the completion of the IPO, subject to the continuous service of the founders through the applicable date. The performance-based condition will be satisfied only if the closing price of the Company’s Class A common stock reaches certain stock-price hurdles that are significantly in excess of the Company’s current valuation over a period of 10 years. The Founder Awards are divided into ten equal tranches with each tranche becoming eligible to vest upon achievement of the specified stock-price hurdles. The estimated amount of unrecognized compensation expense for the Founder Awards as of June 30, 2021 was approximately $100,543, which the Company expects to recognize over a period of 1 year to 7 years, depending upon the achievement of the stock-price hurdles. No compensation expense was recorded as of June 30, 2021 as the performance-based vesting conditions were deemed not probable. Stock option activity as of June 30, 2021 is set forth below: Number of Weighted- Weighted- average remaining contractual life (years) Aggregate intrinsic value Options outstanding at January 1, 2021 8,365 $ 10.68 7.47 $ 230,596 Granted 72 52.80 Exercised (500) 6.62 Repurchased (220) 4.81 Forfeited and expired (51) 13.12 Options outstanding at June 30, 2021 7,666 $ 11.49 7.12 $ 596,343 Options exercisable at June 30, 2021 4,431 $ 7.21 6.09 $ 363,687 The total intrinsic value of options exercised was approximately $17,970 for the period ending June 30, 2021. There were 114 options available for grant at June 30, 2021. RSU activity for the six months ended June 30, 2021 is set forth below: Share units Weighted- Outstanding at January 1, 2021 34 $ 38.08 Granted 561 52.58 Vested — — Forfeited — — Outstanding at June 30, 2021 595 $ 51.76 As of June 30, 2021, there was approximately $23,972 of unrecognized compensation cost related to stock options granted under the plan. That cost is expected to be recognized over a weighted-average period of approximately two years. The amount of unrecognized compensation expense for RSUs as of June 30, 2021 was $30,783 with a weighted average remaining contractual life of four years, for a total unrecognized compensation expense of $54,755. In February of 2021, the Company initiated a tender offer which allowed employees to sell up to 10% of their vested options or shares back to the Company at selling price of $59.77, which was above fair market value of $38.08. The Company paid $13,479 and incurred $5,275 of additional compensation expense related to this tender representing the difference between the aggregate selling price and fair market value of the options and shares sold, and a $7,335 increase to Additional paid-in capital. As a result of this tender, 220 options were put back into the option pool and 23 shares were retired with an $868 increase to Additional paid in capital. Compensation expense is recognized over the service period, which approximates the vesting period. Total compensation expense was $2,907 and $5,458 for the three and six months ended June 30, 2021, respectively and $1,681 and $2,835 or the three and six months ended June 30, 2020, respectively. Stock based compensation expense is included in the consolidated statements of operations as shown in the following table: Three Months Ended June 30, Six Months Ended June 30, 2021 2020 2021 2020 Cost of Revenue $ — $ — $ 2 $ 1 Research and development 1,105 532 2,216 979 Sales and marketing 72 107 140 180 General and administrative 1,730 1,042 3,100 1,675 Total $ 2,907 $ 1,681 $ 5,458 $ 2,835

COMMITMENTS AND CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	6 Months Ended
	Jun. 30, 2021
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	COMMITMENTS AND CONTINGENCIES Legal Proceedings — From time to time, the Company may become involved in various legal proceedings in the ordinary course of its business and may be subject to third-party infringement claims. Sales and use and value-added tax (VAT) — The Company determined that it was required to pay sales and use and VAT taxes in various jurisdictions. The Company is in the process of filing voluntary disclosure agreements with certain jurisdictions and remitting the estimated taxes. If these jurisdictions determine that additional amounts are necessary, the Company will be required to pay accordingly. Related Parties - The Company has determined that there were no transactions with related parties as of or during the three and six months ended June 30, 2021 and 2020.

ACCRUED EXPENSES AND OTHER LIAB

ACCRUED EXPENSES AND OTHER LIABILITIES	6 Months Ended
	Jun. 30, 2021
Other Liabilities Disclosure [Abstract]
ACCRUED EXPENSES AND OTHER LIABILITIES	ACCRUED EXPENSES AND OTHER LIABILITIES Accrued expenses and other liabilities consisted of the following: June 30, December 31, Sales and VAT tax accrual $ 2,597 $ 2,301 Deferred offering costs 1,076 — Marketing related accruals 839 1,513 Employee related benefits 694 889 Obligations under current leases 630 1,111 Contributor awards 577 — Other 3,366 2,820 Total $ 9,779 $ 8,634

EMPLOYEE BENEFIT PLAN

EMPLOYEE BENEFIT PLAN	6 Months Ended
	Jun. 30, 2021
Postemployment Benefits [Abstract]
EMPLOYEE BENEFIT PLAN	EMPLOYEE BENEFIT PLANThe Company sponsors a profit sharing plan with a 401(k) feature, the Duolingo Retirement Plan, (the “Plan”) for eligible employees. The current Plan, effective January 1, 2021, provides for Company safe harbor matching contributions of 100% of the first 4% of the employees’ elective deferrals and 50% of the next 2%, with vesting starting upon the first day of employment. The prior year Plan provided for Company safe harbor matching contributions of 100% of the first 3% of the employees’ elective deferrals and 50% of the next 2%, with vesting starting upon the first day of employment. The Company also has the option to make discretionary matching or profit sharing contributions. The Company made safe harbor matching contributions of approximately $781 and $1,483 for the three and six months ended June 30, 2021, respectively and $472 and $813 for the three and six months ended June 30, 2020, respectively. The Company did not make any discretionary matching or profit sharing contributions during the six months ended June 30, 2021 or 2020.

EARNINGS PER SHARE

EARNINGS PER SHARE	6 Months Ended
	Jun. 30, 2021
Earnings Per Share [Abstract]
EARNINGS PER SHARE	EARNINGS PER SHARE Basic and diluted net loss per share attributable to common stockholders is presented in conformity with the two-class method required for participating securities. The Company considers all series of the convertible preferred stock to be participating securities. Under the two-class method, the net loss attributable to common stockholders is not allocated to the convertible preferred stock as the holders of the convertible preferred stock do not have a contractual obligation to share in the Company’s losses. Basic net loss per share attributable to common stockholders is computed by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period, less shares subject to repurchase. The diluted net loss per share attributable to common stockholders is computed by giving effect to all potential dilutive common stock equivalents outstanding for the period. The Company uses the more dilutive method of calculating the diluted income per share by applying the more dilutive of either (a) the treasury stock method, if-converted method, or (b) the two-class method in its diluted income (loss) per common share calculation. Potentially dilutive shares are determined by applying the treasury stock method to the assumed exercise of outstanding stock options and the assumed vesting of restricted stock. Potentially dilutive shares issuable upon conversion of the Convertible preferred stock are calculated using the if-converted method. Three Months Ended June 30, Six Months Ended June 30, (In thousands, except per share data) 2021 2020 2021 2020 Numerator: Net (loss) income $ (176) $ 40 $ (13,648) $ (2,193) Less: allocation to preferred stockholders — (24) — — Net (loss) income allocated to common stockholders $ (176) $ 16 $ (13,648) $ (2,193) Denominator: Denominator for basic net income per common share - weighted-average shares 13,172 12,439 13,045 12,441 Effect of dilutive securities Convertible preferred stock — — — — Stock options — 3,764 — — Denominator for dilutive net income per common share - weighted-average shares 13,172 16,203 13,045 12,441 Basic loss per common share $ (0.01) $ — $ (1.05) $ (0.18) Diluted loss per common share $ (0.01) $ — $ (1.05) $ (0.18) Since the Company was in a net loss position for the three and six months ended June 30, 2021 and the six months ended June 30, 2020, there is no difference between the number of shares used to calculate basic and diluted loss per share. The potential shares of common stock that were excluded from the computation of diluted net loss per share attributable to common stockholders for the period presented because including them would have been antidilutive are as follows: Three Months Ended June 30, Six Months Ended June 30, 2021 2020 2021 2020 Convertible preferred stock 19,074 — 19,074 18,488 Stock options 4,431 — 4,431 3,764 Total 23,505 — 23,505 22,252 Dilutive shares under the as-converted method(1) — 18,488 — — ________________ (1) The dilutive shares under the as-converted method assume conversion of the Convertible preferred stock and are presented here merely for reference. In a net income position, diluted earnings per share is determined by the more dilutive of the two-class method or the as-converted method.

SUBSEQUENT EVENTS

SUBSEQUENT EVENTS	6 Months Ended
	Jun. 30, 2021
Subsequent Events [Abstract]
SUBSEQUENT EVENTS	SUBSEQUENT EVENTS Subsequent events were evaluated events through August 12, 2021, the date that these unaudited condensed consolidated financial statements were available to be issued. Equity Plans — In July 2021, Duolingo adopted the 2021 Incentive Award Plan (2021 Plan) and the Employee Stock Purchase Plan (ESPP), each of which became effective on July 26, 2021 in connection with the IPO. An aggregate of 7,832 shares and 1,119 shares of Class A common have been reserved for future issuance under the 2021 Plan and ESPP, respectively. Initial Public Offering — On July 30, 2021, Duolingo completed its IPO of 5,872 shares of its Class A common stock at a price to the public of $102.00 per share, 4,466 of which were sold by the Company and 1,406 of which were sold by certain selling stockholders, which includes the exercise in full by the underwriters of their option to purchase from the Company an additional 766 shares of the Company’s Class A common stock. The gross proceeds to the Company from the initial public offering were $455,523, before deducting underwriting discounts and commissions and estimated offering expenses payable by the Company. The Company did not receive any proceeds from the sale of shares of Class A common stock in the offering by the selling stockholders. Immediately prior to the completion of the IPO, all convertible preferred stock outstanding, totaling approximately 19,074 shares, was automatically converted into an equivalent number of shares of Class B common stock on a one-to-one basis and their carrying value of $182,609 was reclassified to stockholders’ equity.

SUMMARY OF SIGNIFICANT ACCOUN_2

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	6 Months Ended
	Jun. 30, 2021
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation—The unaudited condensed consolidated financial statements include the accounts of the Company and subsidiaries over which the Company has control. All intercompany transactions and balances have been eliminated.
Basis of Presentation and Accounting Principles	Basis of Presentation —The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States (“GAAP”) from the Company’s accounting records and reflect the consolidated financial position and results of operations for the six months ended June 30, 2021 and 2020. Unless otherwise specified, all dollar amounts are referred to in thousands. The unaudited condensed consolidated financial statements have been prepared pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”). Certain information and note disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such SEC rules. We believe that the disclosures made are adequate to make the information presented not misleading. In our opinion, all adjustments considered necessary for a fair presentation of the financial statements have been included, and all adjustments are of a normal and recurring nature. We consistently applied the accounting policies consistent with the annual consolidated financial statements elsewhere in this this Quarterly Report on Form 10-Q, in preparing these unaudited condensed consolidated financial statements, with the exception of accounting standard updates described in Note 2, “Recently Adopted Accounting Pronouncements.” These unaudited condensed consolidated financial statements should be read in conjunction with the audited financial statements and the notes for the fiscal year ended December 31, 2020 included in the final prospectus dated as of July 27, 2021 and filed with the SEC, pursuant to Rule 424(b)(4) on July 28, 2021 (“Final Prospectus”). Certain prior period amounts have been reclassified to conform to the current period presentation Accounting Principles —The financial statements and accompanying notes are prepared in accordance with generally accepted accounting principles in the United States of America (GAAP).
Use of Estimates	Use of Estimates— The preparation of unaudited condensed consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Significant estimates and assumptions reflected in the consolidated financial statements include, but are not limited to, useful lives of property and equipment, valuation of deferred tax assets and liabilities, stock-based compensation, common stock valuation, operating lease right-of-use assets and liabilities, capitalization of internally developed software and associated useful lives and contingent liabilities. Actual results may differ materially from such estimates. Management believes that the estimates, and judgments upon which they rely, are reasonable based upon information available to them at the time that these estimates and judgments are made. To the extent that there are material differences between these estimates and actual results, the Company’s consolidated financial statements will be affected.
Deferred Offering Costs	Deferred Offering Costs—Deferred offering costs, which consist of direct incremental legal, accounting, and consulting fees relating to the Initial Public Offering (“IPO”), will be capitalized. The deferred offering costs will be offset against IPO proceeds upon the consummation of the IPO. In the event the planned IPO is terminated, the deferred offering costs will be expensed.
Cash and Cash Equivalents	Cash and Cash Equivalents—Cash consists primarily of cash on hand and bank deposits. Cash equivalents consist primarily of money market accounts with maturities of three months or less at the date of acquisition and are stated at cost, which approximates fair value. The Company maintains cash deposits with financial institutions that may exceed federally insured limits at times.The Money market funds are considered Level 1 financial assets. Level 1 financial assets use inputs that are unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date.
Advertising Costs	Advertising Costs— Advertising costs were approximately $7,685 and $19,751 for the three and six months ended June 30, 2021 and $6,460 and $10,495 for the three and six months ended June 30, 2020, respectively, and are included within Sales and marketing in the unaudited condensed consolidated statement of operations and comprehensive loss.
Income Taxes	Income Taxes— The Company’s period provision for income taxes is computed by using an estimate of the annual effective tax rate, adjusted for discrete items taken into account in the relevant period, if any. Each quarter, the annual effective income tax rate is recomputed and if there are material changes in the estimate, a cumulative adjustment is made.
Contributors	Contributors— On March 10, 2021, the Company announced that it was ending its non-employee volunteer program, which began in 2013 to build and improve language courses. As part of this change, those contributors who participated in the program became eligible to receive a one-time award, up to an aggregate amount of approximately $5,098, including fees paid to process payments of approximately $526. The Company accounted for this under Financial Accounting Standards Board (“FASB”) Accounting Standards Codifications (“ASC”) 958-720, Not-For-Profit Entities - Other Expenses and ASC 720-25, Contributions Made
Concentration of Credit Risk	Concentration of Credit Risk —The Company’s concentration of credit risk relates to financial institutions holding the Company’s cash and cash equivalents and platforms with significant accounts receivable balances and revenue transactions. The Company maintains cash deposits with financial institutions that may exceed federally insured limits at times. Management believes that the financial institutions that hold the Company’s deposits are financially credit worthy and, accordingly, minimal credit risk exists with respect to those balances. The majority of our revenue comes through our subscriptions and advertising streams and payments are made to Duolingo through service providers. The top two, Apple and Google, accounted for 66.4% and 18.3% of total accounts receivable as of June 30, 2021, respectively. The top three service providers, Apple, Google and Stripe, accounted for 47.8%, 28.9% and 13.8% of total accounts receivable as of December 31, 2020, respectively. Apple, Google, and Stripe, processed 51.6%, 29.3% and 10.3% and 50.9%, 28.8%, and 10.3% of total revenue for the three and six months ended June 30, 2021, respectively. Three service providers, Apple, Google, and Stripe, processed 50.0%, 26.5%, and 10.1% of total revenue for the three months ended June 30, 2020, respectively. Two services providers, Apple and Google, processed 52.1% and 27.6% for the six months ended June 30, 2020.
Recently Adopted Accounting Pronouncements/Recently Issued Pronouncements Not Yet Adopted	Recently Adopted Accounting Pronouncements In August 2018, the FASB issued Accounting Standard Update (“ASU”) No. 2018-15, Intangibles-Goodwill and Other-Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That is a Service Contract (ASU 2018-15) . ASU 2018-15 aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. The Company adopted this guidance on January 1, 2021 and it did not have a material impact on its unaudited condensed consolidated financial statements and related disclosures upon adoption. Recently Issued Pronouncements Not Yet Adopted In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes
Revenue	The Company has three predominant sources of revenue; time-based subscriptions, in-app advertising placement by third parties and the Duolingo English Test. Revenue is recognized upon transfer of control of promised products or services to users in an amount that reflects the consideration the Company expects to receive in exchange for those services. The Company does not enter into contracts with a customer that contain multiple promises that result in multiple performance obligations. Revenue is recorded net of taxes, assessed by a government authority that are both imposed on and concurrent with specific revenue transactions between us and our users. Revenue from time-based subscriptions includes a stand-ready obligation to provide hosting services that are consumed by the customer over the subscription period. Users can purchase Duolingo monthly or they can purchase a six-month or year-long subscription and pay for the subscription at the time of purchase. Therefore, such payments are initially recorded to deferred revenue. The user has the ability to download limited content offline. However, as there is a significant level of integration and interdependency with the online functionality, the Company considers the service to be a single performance obligation for the online and offline content. The Company enters into arrangements with advertising networks to monetize the in-app advertising inventory. Revenue from in-app advertising placement is recognized at a point in time when the advertisement is placed and is based upon the amount received. Duolingo English Test revenue is generally recognized once the tests have gone through the proctoring process and a certification decision has been made. This process usually takes less than 48 hours after the test has been completed and uploaded. Customers have 90 days from the date of purchase to take the exam or their purchase will expire and revenue will be recognized. Virtually all customers complete their exams prior to expiration. Sometimes organizations may purchase tests in bulk via coupons with a one year expiration date. The company will defer revenue from all tests that haven’t been proctored nor expired. The Company’s users have the option to purchase consumable in-app virtual goods. The Company recognizes revenue over the period in which the user consumes the virtual good, which is generally within a month. Principal Agent Considerations —The Company makes its application available to be downloaded through third-party digital distribution service providers. Users who purchase subscriptions also pay through the respective app stores. The Company evaluates the purchases via third-party payment processors to determine whether its revenues should be reported gross or net of fees retained by the payment processor. The Company is the principal in the transaction with the end user as a result of controlling, hosting, and integrating the delivery of the virtual items to the end user. The Company records revenue gross as a principal and records fees paid to third-party payment processors as Cost of revenues. Contract Balances —Deferred revenue mostly consists of payments we receive in advance of revenue recognition, and is mostly related to time-based subscriptions, which will be recognized into revenue over the course of the upcoming year, which is 12 months or less. Additionally, Duolingo English Test has deferred revenue related to tests that have been purchased, but will not be recognized until the tests have been proctored. Disaggregation of Revenue In accordance with ASC 606, Revenue from Contracts with Customers , the Company disaggregates revenue from contracts with customers into source of revenue, which most closely depicts how the nature, amount, timing and uncertainty of revenue and cash flows are affected by economic factors.

SUMMARY OF SIGNIFICANT ACCOUN_3

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)	6 Months Ended
	Jun. 30, 2021
Accounting Policies [Abstract]
Schedule of Cash and Cash Equivalents	The following table shows the breakout between cash and money market funds. June 30, December 31, Cash $ 10,571 $ 20,428 Money market funds 104,065 100,062 Total $ 114,636 $ 120,490

REVENUE (Tables)

REVENUE (Tables)	6 Months Ended
	Jun. 30, 2021
Revenue from Contract with Customer [Abstract]
Disaggregation of Revenue	Information regarding source of revenues: Three Months Ended June 30, Six Months Ended June 30, 2021 2020 2021 2020 Over time $ 43,502 $ 28,274 $ 83,557 $ 50,445 Point in time 15,301 11,737 30,606 17,678 Total revenue $ 58,803 $ 40,011 $ 114,163 $ 68,123 Information regarding revenue by stream: Three Months Ended June 30, Six Months Ended June 30, 2021 2020 2021 2020 Revenues: Subscription $ 43,502 $ 28,274 $ 83,557 $ 50,445 Advertising 9,056 6,808 18,331 11,816 Duolingo English Test 4,833 4,598 9,868 5,351 Other (1) 1,412 331 2,407 511 Total revenues $ 58,803 $ 40,011 $ 114,163 $ 68,123 ________________ (1) Other revenue is mainly comprised of in app purchases of virtual goods.
Contract with Customer, Contract Asset, Contract Liability, and Receivable	Changes in deferred revenues were as follows: Six Months Ended June 30, 2021 2020 Beginning balance—January 1 $ 54,792 $ 26,307 Amount from beginning balance recognized into revenue (41,498) (19,833) Recognition of deferred revenue (45,378) (30,737) Deferral of revenue 103,081 68,897 Ending balance—June 30 $ 70,997 $ 44,634

PROPERTY and EQUIPMENT, net (Ta

PROPERTY and EQUIPMENT, net (Tables)	6 Months Ended
	Jun. 30, 2021
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	Property and equipment consists of the following as of June 30, 2021 and December 31, 2020: June 30, December 31, Leasehold improvements $ 9,336 $ 7,536 Furniture, fixtures and equipment 2,364 1,959 Total property and equipment 11,700 9,495 Less: accumulated depreciation (4,008) (3,067) Total property and equipment, net $ 7,692 $ 6,428

Intangible Assets, Goodwill and

Intangible Assets, Goodwill and Other (Tables)	6 Months Ended
	Jun. 30, 2021
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Finite-Lived Intangible Assets	Capitalized software consists of the following as of June 30, 2021 and December 31, 2020: June 30, December 31, Capitalized software $ 9,912 $ 8,181 Less: accumulated amortization (6,181) (5,885) Capitalized software, net $ 3,731 $ 2,296
Finite-lived Intangible Assets Amortization Expense	Amortization expense is included within the following financial statement line items within the Company’s consolidated statement of operations: Three Months Ended June 30, Six Months Ended June 30, 2021 2020 2021 2020 Cost of revenue $ — $ 25 $ — $ 51 Sales and marketing 148 218 296 251 Total $ 148 $ 243 $ 296 $ 302

CONVERTIBLE PREFERRED STOCK (Ta

CONVERTIBLE PREFERRED STOCK (Tables)	6 Months Ended
	Jun. 30, 2021
Temporary Equity Disclosure [Abstract]
Temporary Equity	Convertible preferred stock is comprised of the following as of June 30, 2021 and December 31, 2020: Shares Series Authorized Outstanding Per share price Aggregate liquidation preference Funds received Fees incurred Carrying value of convertible preferred stock A 3,865 3,865 $ 0.85 $ 3,300 $ 3,300 $ 52 $ 3,248 B 6,298 6,298 2.38 15,000 15,000 60 14,940 C 2,948 2,948 6.78 20,000 20,000 112 19,888 D 3,154 3,154 14.27 45,000 45,000 146 44,853 E 1,224 1,224 20.43 25,000 25,000 92 24,909 F 758 758 39.57 30,000 30,000 153 29,848 G 241 241 41.38 10,000 10,000 24 9,976 H 586 586 59.77 35,000 35,000 52 34,947 Total 19,074 19,074 $ 183,300 $ 183,300 $ 691 $ 182,609

STOCK-BASED COMPENSATION (Table

STOCK-BASED COMPENSATION (Tables)	6 Months Ended
	Jun. 30, 2021
Equity [Abstract]
Share-based Payment Arrangement, Option, Activity	Stock option activity as of June 30, 2021 is set forth below: Number of Weighted- Weighted- average remaining contractual life (years) Aggregate intrinsic value Options outstanding at January 1, 2021 8,365 $ 10.68 7.47 $ 230,596 Granted 72 52.80 Exercised (500) 6.62 Repurchased (220) 4.81 Forfeited and expired (51) 13.12 Options outstanding at June 30, 2021 7,666 $ 11.49 7.12 $ 596,343 Options exercisable at June 30, 2021 4,431 $ 7.21 6.09 $ 363,687
Share-based Payment Arrangement, Restricted Stock Unit, Activity	RSU activity for the six months ended June 30, 2021 is set forth below: Share units Weighted- Outstanding at January 1, 2021 34 $ 38.08 Granted 561 52.58 Vested — — Forfeited — — Outstanding at June 30, 2021 595 $ 51.76
Disclosure of Share-based Compensation Arrangements by Share-based Payment Award	Stock based compensation expense is included in the consolidated statements of operations as shown in the following table: Three Months Ended June 30, Six Months Ended June 30, 2021 2020 2021 2020 Cost of Revenue $ — $ — $ 2 $ 1 Research and development 1,105 532 2,216 979 Sales and marketing 72 107 140 180 General and administrative 1,730 1,042 3,100 1,675 Total $ 2,907 $ 1,681 $ 5,458 $ 2,835

ACCRUED EXPENSES AND OTHER LI_2

ACCRUED EXPENSES AND OTHER LIABILITIES (Tables)	6 Months Ended
	Jun. 30, 2021
Other Liabilities Disclosure [Abstract]
Schedule of Accrued Liabilities and Other Liabilities	Accrued expenses and other liabilities consisted of the following: June 30, December 31, Sales and VAT tax accrual $ 2,597 $ 2,301 Deferred offering costs 1,076 — Marketing related accruals 839 1,513 Employee related benefits 694 889 Obligations under current leases 630 1,111 Contributor awards 577 — Other 3,366 2,820 Total $ 9,779 $ 8,634

EARNINGS PER SHARE (Tables)

EARNINGS PER SHARE (Tables)	6 Months Ended
	Jun. 30, 2021
Earnings Per Share [Abstract]
Schedule of Earnings Per Share, Basic and Diluted	Three Months Ended June 30, Six Months Ended June 30, (In thousands, except per share data) 2021 2020 2021 2020 Numerator: Net (loss) income $ (176) $ 40 $ (13,648) $ (2,193) Less: allocation to preferred stockholders — (24) — — Net (loss) income allocated to common stockholders $ (176) $ 16 $ (13,648) $ (2,193) Denominator: Denominator for basic net income per common share - weighted-average shares 13,172 12,439 13,045 12,441 Effect of dilutive securities Convertible preferred stock — — — — Stock options — 3,764 — — Denominator for dilutive net income per common share - weighted-average shares 13,172 16,203 13,045 12,441 Basic loss per common share $ (0.01) $ — $ (1.05) $ (0.18) Diluted loss per common share $ (0.01) $ — $ (1.05) $ (0.18)
Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share	The potential shares of common stock that were excluded from the computation of diluted net loss per share attributable to common stockholders for the period presented because including them would have been antidilutive are as follows: Three Months Ended June 30, Six Months Ended June 30, 2021 2020 2021 2020 Convertible preferred stock 19,074 — 19,074 18,488 Stock options 4,431 — 4,431 3,764 Total 23,505 — 23,505 22,252 Dilutive shares under the as-converted method(1) — 18,488 — — ________________ (1) The dilutive shares under the as-converted method assume conversion of the Convertible preferred stock and are presented here merely for reference. In a net income position, diluted earnings per share is determined by the more dilutive of the two-class method or the as-converted method.

SUMMARY OF SIGNIFICANT ACCOUN_4

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 114,636	$ 120,490
Cash
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	10,571	20,428
Money market funds
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 104,065	$ 100,062

SUMMARY OF SIGNIFICANT ACCOUN_5

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) - USD ($) $ in Thousands	Mar. 10, 2021	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Concentration Risk [Line Items]
Deferred offering costs		$ 2,627		$ 2,627		$ 0
Deferred offering costs incurred but not yet paid				1,076	$ 0
Advertising expense		7,685	$ 6,460	19,751	$ 10,495
Volunteer program, one-time payment to contributors	$ 5,098	4,220		4,220
Volunteer program, transaction fees	$ 526
Volunteer program award reserve		(878)
Contributor awards		$ 577		$ 577		$ 0
Accounts Receivable | Customer Concentration Risk | Google
Concentration Risk [Line Items]
Concentration risk, percentage				66.40%		28.90%
Accounts Receivable | Customer Concentration Risk | Apple
Concentration Risk [Line Items]
Concentration risk, percentage				18.30%		47.80%
Accounts Receivable | Customer Concentration Risk | Stripe
Concentration Risk [Line Items]
Concentration risk, percentage						13.80%
Revenue Benchmark | Customer Concentration Risk | Google
Concentration Risk [Line Items]
Concentration risk, percentage		29.30%	26.50%	28.80%	27.60%
Revenue Benchmark | Customer Concentration Risk | Apple
Concentration Risk [Line Items]
Concentration risk, percentage		51.60%	50.00%	50.90%	52.10%
Revenue Benchmark | Customer Concentration Risk | Stripe
Concentration Risk [Line Items]
Concentration risk, percentage		10.30%	10.10%	10.30%

REVENUE - Disaggregation of Rev

REVENUE - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 58,803	$ 40,011	$ 114,163	$ 68,123
Subscription
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	43,502	28,274	83,557	50,445
Advertising
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	9,056	6,808	18,331	11,816
Duolingo English Test
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	4,833	4,598	9,868	5,351
Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,412	331	2,407	511
Over time
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	43,502	28,274	83,557	50,445
Point in time
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 15,301	$ 11,737	$ 30,606	$ 17,678

REVENUE - Deferred Revenue Roll

REVENUE - Deferred Revenue Rollforward (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Revenue from Contract with Customer, Liability [Roll Forward]
Beginning balance	$ 54,792	$ 26,307
Amount from beginning balance recognized into revenue	(41,498)	(19,833)
Recognition of deferred revenue	(45,378)	(30,737)
Deferral of revenue	103,081	68,897
Ending balance	$ 70,997	$ 44,634

PROPERTY and EQUIPMENT, net (De

PROPERTY and EQUIPMENT, net (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Property, plant, and equipment, gross	$ 11,700		$ 11,700		$ 9,495
Less: accumulated depreciation	(4,008)		(4,008)		(3,067)
Total property and equipment, net	7,692		7,692		6,428
Depreciation	488	$ 378	940	$ 720
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, plant, and equipment, gross	9,336		9,336		7,536
Furniture, fixtures and equipment
Property, Plant and Equipment [Line Items]
Property, plant, and equipment, gross	$ 2,364		$ 2,364		$ 1,959

CAPITALIZED SOFTWARE, net - Cap

CAPITALIZED SOFTWARE, net - Capitalized Software (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Capitalized software	$ 9,912	$ 8,181
Less: accumulated amortization	(6,181)	(5,885)
Finite-Lived Intangible Assets, Net	$ 3,731	$ 2,296

CAPITALIZED SOFTWARE, net - Nar

CAPITALIZED SOFTWARE, net - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Amortization of capitalized computer software	$ 148	$ 243	$ 296	$ 302

CAPITALIZED SOFTWARE, net - Sch

CAPITALIZED SOFTWARE, net - Schedule of Capitalized Software Amortization Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Finite-Lived Intangible Assets [Line Items]
Amortization of capitalized computer software	$ 148	$ 243	$ 296	$ 302
Cost of Revenue
Finite-Lived Intangible Assets [Line Items]
Amortization of capitalized computer software	0	25	0	51
Sales and marketing
Finite-Lived Intangible Assets [Line Items]
Amortization of capitalized computer software	$ 148	$ 218	$ 296	$ 251

INCOME TAXES (Details)

INCOME TAXES (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Income Tax Disclosure [Abstract]
Effective income tax rate reconciliation, percent	(0.60%)	21.60%	(0.10%)	(1.00%)
Provision for income taxes	$ 1	$ 11	$ 18	$ 22
Pretax income (loss)	$ (175)	$ 51	$ (13,630)	$ (2,171)

CONVERTIBLE PREFERRED STOCK - S

CONVERTIBLE PREFERRED STOCK - Schedule of Convertible Preferred Stock (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Temporary Equity [Line Items]
Authorized (in shares)	19,074		19,074
Outstanding (in shares)	19,074	19,074	19,074	18,488	18,247	18,247
Aggregate liquidation preference	$ 183,300		$ 183,300
Funds received	183,300		183,300
Fees incurred	691		691
Carrying value of convertible preferred stock	$ 182,609	$ 182,609	$ 182,609	$ 147,662	$ 137,686	$ 137,686
Series A Preferred Stock
Temporary Equity [Line Items]
Authorized (in shares)	3,865		3,865
Outstanding (in shares)	3,865		3,865
Per share price at issuance (in usd per share)	$ 0.85		$ 0.85
Aggregate liquidation preference	$ 3,300		$ 3,300
Funds received	3,300		3,300
Fees incurred	52		52
Carrying value of convertible preferred stock	$ 3,248		$ 3,248
Series B Preferred Stock
Temporary Equity [Line Items]
Authorized (in shares)	6,298		6,298
Outstanding (in shares)	6,298		6,298
Per share price at issuance (in usd per share)	$ 2.38		$ 2.38
Aggregate liquidation preference	$ 15,000		$ 15,000
Funds received	15,000		15,000
Fees incurred	60		60
Carrying value of convertible preferred stock	$ 14,940		$ 14,940
Series C Preferred Stock
Temporary Equity [Line Items]
Authorized (in shares)	2,948		2,948
Outstanding (in shares)	2,948		2,948
Per share price at issuance (in usd per share)	$ 6.78		$ 6.78
Aggregate liquidation preference	$ 20,000		$ 20,000
Funds received	20,000		20,000
Fees incurred	112		112
Carrying value of convertible preferred stock	$ 19,888		$ 19,888
Series D Preferred Stock
Temporary Equity [Line Items]
Authorized (in shares)	3,154		3,154
Outstanding (in shares)	3,154		3,154
Per share price at issuance (in usd per share)	$ 14.27
Aggregate liquidation preference	$ 45,000		$ 45,000
Funds received	45,000		45,000
Fees incurred	146		146
Carrying value of convertible preferred stock	$ 44,853		$ 44,853
Series E Preferred Stock
Temporary Equity [Line Items]
Authorized (in shares)	1,224		1,224
Outstanding (in shares)	1,224		1,224
Per share price at issuance (in usd per share)	$ 20.43		$ 20.43
Aggregate liquidation preference	$ 25,000		$ 25,000
Funds received	25,000		25,000
Fees incurred	92		92
Carrying value of convertible preferred stock	$ 24,909		$ 24,909
Series F Preferred Stock
Temporary Equity [Line Items]
Authorized (in shares)	758
Outstanding (in shares)	758		758
Per share price at issuance (in usd per share)	$ 39.57		$ 39.57
Aggregate liquidation preference	$ 30,000		$ 30,000
Funds received	30,000		30,000
Fees incurred	153		153
Carrying value of convertible preferred stock	$ 29,848		$ 29,848
Series G Preferred Stock
Temporary Equity [Line Items]
Authorized (in shares)	241		241
Outstanding (in shares)	241		241
Per share price at issuance (in usd per share)	$ 41.38		$ 41.38
Aggregate liquidation preference	$ 10,000		$ 10,000
Funds received	10,000		10,000
Fees incurred	24		24
Carrying value of convertible preferred stock	$ 9,976		$ 9,976
Series H Preferred Stock
Temporary Equity [Line Items]
Authorized (in shares)	586		586
Outstanding (in shares)	586		586
Per share price at issuance (in usd per share)	$ 59.77		$ 59.77
Aggregate liquidation preference	$ 35,000		$ 35,000
Funds received	35,000		35,000
Fees incurred	52		52
Carrying value of convertible preferred stock	$ 34,947		$ 34,947

CONVERTIBLE PREFERRED STOCK - N

CONVERTIBLE PREFERRED STOCK - Narrative (Details)	Jun. 30, 2021USD ($)
Temporary Equity Disclosure [Abstract]
Threshold proceeds from a public offering to trigger mandatory conversion of convertible preferred stock	$ 50,000,000
Threshold percent of preferred stock holders electing to convert to trigger a mandatory conversion of convertible preferred stock	65.00%

STOCK-BASED COMPENSATION - Narr

STOCK-BASED COMPENSATION - Narrative (Details) $ / shares in Units, shares in Thousands, $ in Thousands	1 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2021USD ($)trancheshares	Feb. 28, 2021USD ($)$ / sharesshares	Jun. 30, 2021USD ($)trancheshares	Jun. 30, 2020USD ($)	Jun. 30, 2021USD ($)trancheshares	Jun. 30, 2020USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate intrinsic value, exercised					$ 17,970
Unrecognized compensation cost, options	$ 23,972		$ 23,972		23,972
Unrecognized compensation cost	$ 54,755		54,755		54,755
Fair market value of share price (in usd per share) | $ / shares		$ 38.08
Payments for repurchase of stock options					7,335	$ 0
Compensation expense			$ 2,907	$ 1,681	5,458	$ 2,835
Options repurchased					7,335
Common stock repurchased and retired					868
Additional Paid-in Capital
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options repurchased					7,335
Common stock repurchased and retired					$ 868
Tender Offer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Sale of stock, price per share (in usd per share) | $ / shares		$ 59.77
Payments for repurchase of stock options		$ 13,479
Compensation expense		$ 5,275
Options recycled back into option pool (in shares) | shares		220
Common stock repurchased and retired (in shares) | shares		23
Common stock repurchased and retired		$ 868
Tender Offer | Additional Paid-in Capital
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options repurchased		$ 7,335
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost, weighted-average period of recognition					2 years
Number of options available for grant (in shares) | shares	114		114		114
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity instruments other than options, grants in period (in shares) | shares					561
Unrecognized compensation cost, weighted-average period of recognition					4 years
Unrecognized compensation expense, excluding options	$ 30,783		$ 30,783		$ 30,783
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense, excluding options	$ 100,543		$ 100,543		$ 100,543
Performance Shares | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost, weighted-average period of recognition	1 year
Performance Shares | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost, weighted-average period of recognition	7 years
Performance Shares | Founders
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity instruments other than options, grants in period (in shares) | shares	1,800
Settlement period after vesting	1 year
Award vesting percentage	25.00%
Award service period	10 years
Number of tranches | tranche	10		10		10
Compensation expense					$ 0
2011 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement, number of shares authorized (in shares) | shares	10,113		10,113		10,113
Vesting period					4 years
Share-based compensation, term of award					10 years
Incentive stock options, minimum exercise price, percent					100.00%
2011 Equity Incentive Plan | Principal Owner
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Incentive stock options, minimum exercise price, percent					110.00%

STOCK-BASED COMPENSATION - Stoc

STOCK-BASED COMPENSATION - Stock Option Activity (Details) $ / shares in Units, shares in Thousands, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2021USD ($)$ / sharesshares	Dec. 31, 2020USD ($)$ / sharesshares
Number of options
Outstanding, beginning balance (in shares) | shares	8,365
Granted (in shares) | shares	72
Exercised (in shares) | shares	(500)
Repurchased (in shares) | shares	(220)
Forfeited and expired (in shares) | shares	(51)
Outstanding, ending balance (in shares) | shares	7,666	8,365
Number of shares exercisable (in shares) | shares	4,431
Weighted Average Exercise Price
Outstanding, beginning balance (in usd per share) | $ / shares	$ 10.68
Granted (in usd per share) | $ / shares	52.80
Exercised (in usd per share) | $ / shares	6.62
Repurchased (in usd per share) | $ / shares	4.81
Forfeited and expired (in usd per share) | $ / shares	13.12
Outstanding, ending balance (in usd per share) | $ / shares	11.49	$ 10.68
Weighted average exercise price, exercisable (in usd per share) | $ / shares	$ 7.21
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted average remaining contractual term (in years), outstanding	7 years 1 month 13 days	7 years 5 months 19 days
Weighted average remaining contractual term (in years), exercisable	6 years 1 month 2 days
Aggregate intrinsic value, outstanding | $	$ 596,343	$ 230,596
Aggregate intrinsic value, exercisable | $	$ 363,687

STOCK-BASED COMPENSATION - Sche

STOCK-BASED COMPENSATION - Schedule of Restricted Stock Unit Activity (Details) - Restricted Stock Units (RSUs) shares in Thousands	6 Months Ended
	Jun. 30, 2021$ / sharesshares	Dec. 31, 2020shares
Share units
Beginning balance (in shares) | shares	595	34
Granted (in shares) | shares	561
Vested (in shares) | shares	0
Forfeited (in shares) | shares	0
Ending balance (in shares) | shares	595
Weighted- average grant date fair value per share
Beginning balance (in usd per share) | $ / shares	$ 38.08
Granted (in usd per share) | $ / shares	52.58
Vested (in usd per share) | $ / shares	0
Forfeited (in shares) | $ / shares	0
Ending balance (in usd per share) | $ / shares	$ 51.76

STOCK-BASED COMPENSATION - Sc_2

STOCK-BASED COMPENSATION - Schedule of Income Statement Location for Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$ 2,907	$ 1,681	$ 5,458	$ 2,835
Cost of Revenue
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	0	0	2	1
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	1,105	532	2,216	979
Sales and marketing
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	72	107	140	180
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$ 1,730	$ 1,042	$ 3,100	$ 1,675

ACCRUED EXPENSES AND OTHER LI_3

ACCRUED EXPENSES AND OTHER LIABILITIES (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Other Liabilities Disclosure [Abstract]
Sales and VAT tax accrual	$ 2,597	$ 2,301
Deferred offering costs	1,076	0
Marketing related accruals	839	1,513
Employee related benefits	$ 694	$ 889
Operating lease, liability, current, statement of financial position, extensible enumeration	Total	Total
Obligations under current leases	$ 630	$ 1,111
Contributor awards	577	0
Other	3,366	2,820
Total	$ 9,779	$ 8,634

EMPLOYEE BENEFIT PLAN (Details)

EMPLOYEE BENEFIT PLAN (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, contributions by employer	$ 781	$ 472	$ 1,483	$ 813
Percent of Employee Gross Pay with 100% Matching Contribution
Defined Benefit Plan Disclosure [Line Items]
Employer matching contribution, percent of match			100.00%	100.00%
Employer matching contribution, percent of employees' gross pay			4.00%	3.00%
Percent of Employee Gross Pay with 50% Matching Contribution
Defined Benefit Plan Disclosure [Line Items]
Employer matching contribution, percent of match			50.00%	50.00%
Employer matching contribution, percent of employees' gross pay			2.00%	2.00%

EARNINGS PER SHARE - Schedule o

EARNINGS PER SHARE - Schedule of Earnings Per Share, Basic and Diluted (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Earnings Per Share [Abstract]
Net (loss) income	$ (176)	$ 40	$ (13,648)	$ (2,193)
Less: allocation to preferred stockholders	0	(24)	0	0
Net (loss) income allocated to common stockholders, basic	(176)	16	(13,648)	(2,193)
Net (loss) income allocated to common stockholders, diluted	$ (176)	$ 16	$ (13,648)	$ (2,193)
Denominator for basic net income per common share - weighted-average shares (in shares)	13,172	12,439	13,045	12,441
Effect of dilutive securities
Convertible preferred stock (in shares)	0	0	0	0
Stock options (in shares)	0	3,764	0	0
Denominator for dilutive net income per common share - weighted-average shares (in shares)	13,172	16,203	13,045	12,441
Basic loss per common share (in usd per share)	$ (0.01)	$ 0	$ (1.05)	$ (0.18)
Diluted loss per common share (in usd per share)	$ (0.01)	$ 0	$ (1.05)	$ (0.18)

EARNINGS PER SHARE - Schedule_2

EARNINGS PER SHARE - Schedule of Antidilutive Securities (Details) - shares shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from the computation of earnings per share (in shares)	23,505	0	23,505	22,252
Dilutive shares under the as-converted method (in shares)	0	18,488	0	0
Convertible preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from the computation of earnings per share (in shares)	19,074	0	19,074	18,488
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from the computation of earnings per share (in shares)	4,431	0	4,431	3,764

SUBSEQUENT EVENTS (Details)

SUBSEQUENT EVENTS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	Jul. 30, 2021	Jun. 30, 2021	Jun. 30, 2020	Jul. 26, 2021	Dec. 31, 2020
Subsequent Event [Line Items]
Common stock issued (in shares)		$ 1			$ 1
Deferred offering costs		2,627			$ 0
Deferred offering costs incurred but not yet paid		$ 1,076	$ 0
Subsequent Event
Subsequent Event [Line Items]
Proceeds from Issuance of initial public offering	$ 455,523
Deferred Offering Costs Reclassified to Stockholders Equity	$ 4,000
Common Class A | Subsequent Event | 2021 Plan
Subsequent Event [Line Items]
Common stock reserved for future issuance (in shares)				7,832
Common Class A | Subsequent Event | Employee Stock Purchase Plan
Subsequent Event [Line Items]
Common stock reserved for future issuance (in shares)				1,119
Common Class A | Subsequent Event | IPO
Subsequent Event [Line Items]
Sale of stock, number of shares issued in transaction (in shares)	5,872
Sale of stock, price per share (in usd per share)	$ 102
Common Class A | Subsequent Event | IPO | Duolingo
Subsequent Event [Line Items]
Sale of stock, number of shares issued in transaction (in shares)	4,466
Common Class A | Subsequent Event | IPO | Stockholders
Subsequent Event [Line Items]
Sale of stock, number of shares issued in transaction (in shares)	1,406
Common Class A | Subsequent Event | Over-Allotment Option
Subsequent Event [Line Items]
Sale of stock, number of shares issued in transaction (in shares)	766
Common Class B | Subsequent Event
Subsequent Event [Line Items]
Convertible preferred stock, shares issued upon conversion (in shares)	19,074
Common stock issued (in shares)	$ 182,609