What size duct for 150 CFM?
6 inch round — to stay quiet (900 fpm or below).
If commercial velocity is acceptable (up to 1500 fpm), 5 in will pass it — but at 1,100 fpm, which is audible in a home. 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.
150 CFM in each candidate size
| Round size | Velocity | Friction / 100 ft | Verdict |
|---|---|---|---|
| 5 in | 1,100 fpm | 0.419 in. w.c. | Runs fast |
| 6 in ← recommended | 764 fpm | 0.171 in. w.c. | Within range |
| 7 in | 561 fpm | 0.081 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 150 CFM
If 6 in round will not fit the cavity, these carry the same air at the same friction loss.
| Shape | Size | Aspect ratio |
|---|---|---|
| Round | 6 in | 1.0 |
| Flat oval | 8 × 7 in | 1.1 |
What moves 150 CFM
- Bedroom supply
- Small range hood
- Single large register
The catch at 150 CFM
150 CFM is where 6 in becomes the honest minimum. It will physically pass through 5 in, but at a velocity that most homeowners describe as whistling.
Working the other way round? See how many CFM a 6 in duct carries, or size a real system with the calculator.