| Cover | Nov. 08, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Nov. 8, 2021 |
| Entity Registrant Name | Castle Biosciences, Inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-38984 |
| Entity Tax Identification Number | 77-0701774 |
| Entity Address, Address Line One | 505 S. Friendswood Drive |
| Entity Address, Address Line Two | Suite 401 |
| Entity Address, City or Town | Friendswood |
| Entity Address, State or Province | TX |
| Entity Address, Postal Zip Code | 77546 |
| City Area Code | 866 |
| Local Phone Number | 788-9007 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $0.001 par value per share |
| Trading Symbol | CSTL |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Entity Central Index Key | 0001447362 |
| Amendment Flag | false |
