What size duct for 50 CFM?
4 inch round — to stay quiet (900 fpm or below).
At commercial velocities the answer is the same 4 in — this airflow does not sit near a size boundary. Which band applies is a comfort decision, not a code requirement, and it is why two calculators can give different answers for the same airflow.
Engineering estimate. Clean galvanized round duct, standard air, straight duct only. Fittings and flex change these numbers substantially — verify against a full Manual D before fabricating. See disclaimer.
50 CFM in each candidate size
| Round size | Velocity | Friction / 100 ft | Verdict |
|---|---|---|---|
| 4 in ← recommended | 573 fpm | 0.169 in. w.c. | Within range |
| 5 in | 367 fpm | 0.057 in. w.c. | Within range |
Sizes step up to the next manufactured diameter — you cannot buy an arbitrary size. Residential designs usually target about 0.1 in. w.c. per 100 ft; if the friction column reads much above that on a long run, size up regardless of the velocity verdict.
Rectangular and oval options for 50 CFM
If 4 in round will not fit the cavity, these carry the same air at the same friction loss.
| Shape | Size | Aspect ratio |
|---|---|---|
| Round | 4 in | 1.0 |
| Flat oval | 8 × 7 in | 1.1 |
What moves 50 CFM
- Small bath exhaust fan
- Single small register
- Powder-room ventilation
The catch at 50 CFM
50 CFM is the code minimum for an intermittent bath fan, and it is the one airflow where 4 in duct is genuinely adequate — provided the run is short and the elbows are few.
Working the other way round? See how many CFM a 4 in duct carries, or size a real system with the calculator.