| Document and Entity Information | Oct. 07, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001742089 |
| Document Type | 8-K |
| Document Period End Date | Oct. 7, 2021 |
| Entity Registrant Name | CBM BANCORP, INC. |
| Entity Incorporation State Country Code | MD |
| Entity File Number | 001-38680 |
| Entity Tax Identification Number | 83-1095537 |
| Entity Address, Address Line One | 2001 East Joppa Road |
| Entity Address, City or Town | Baltimore |
| Entity Address, State or Province | MD |
| Entity Address, Postal Zip Code | 21234 |
| City Area Code | (410) |
| Local Phone Number | 665-7600 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Security 12b Title | Common Stock (par value $0.01 per share) |
| Trading Symbol | CBMB |
| Security Exchange Name | NASDAQ |
