| Cover | Aug. 10, 2021 |
| Cover [Abstract] | |
| Entity Registrant Name | SMITH-MIDLAND CORPORATION |
| Entity Central Index Key | 0000924719 |
| Document Type | 8-K |
| Amendment Flag | false |
| Entity Emerging Growth Company | false |
| Document Period End Date | Aug. 10, 2021 |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 1-13752 |
| Entity Tax Identification Number | 54-1727060 |
| Entity Address Address Line 1 | P.O. Box 300 |
| Entity Address Address Line 2 | 5119 Catlett Road |
| Entity Address City Or Town | Midland |
| Entity Address State Or Province | VA |
| Entity Address Postal Zip Code | 22728 |
| City Area Code | 540 |
| Local Phone Number | 439-3266 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, $0.01 par value per share |
| Trading Symbol | SMID |
| Security Exchange Name | NASDAQ |
