What size duct for 250 CFM?
8 inch round — to stay quiet (900 fpm or below).
If commercial velocity is acceptable (up to 1500 fpm), 6 in will pass it — but at 1,273 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. For scale, 250 CFM is roughly 0.63 tons of cooling at the usual 400 CFM per ton.
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.
250 CFM in each candidate size
| Round size | Velocity | Friction / 100 ft | Verdict |
|---|---|---|---|
| 6 in | 1,273 fpm | 0.438 in. w.c. | Runs fast |
| 8 in ← recommended | 716 fpm | 0.106 in. w.c. | Within range |
| 9 in | 566 fpm | 0.060 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 250 CFM
If 8 in round will not fit the cavity, these carry the same air at the same friction loss.
| Shape | Size | Aspect ratio |
|---|---|---|
| Round | 8 in | 1.0 |
| Rectangular | × in | NaN |
| Flat oval | 10 × 6 in | 1.7 |
What moves 250 CFM
- Large room supply
- Range hood
- Small zone trunk
The catch at 250 CFM
250 CFM is right at the practical ceiling for 7 in in the residential band. Above it, 8 in is not optional if the run is to stay quiet.
Working the other way round? See how many CFM a 8 in duct carries, or size a real system with the calculator.