What size duct for 300 CFM?
8 inch round — to stay quiet (900 fpm or below).
If commercial velocity is acceptable (up to 1500 fpm), 7 in will pass it — but at 1,123 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, 300 CFM is roughly 0.75 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.
300 CFM in each candidate size
| Round size | Velocity | Friction / 100 ft | Verdict |
|---|---|---|---|
| 7 in | 1,123 fpm | 0.286 in. w.c. | Runs fast |
| 8 in ← recommended | 859 fpm | 0.148 in. w.c. | Within range |
| 9 in | 679 fpm | 0.083 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 300 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 300 CFM
- Large living area
- Powerful range hood
- Small trunk run
The catch at 300 CFM
300 CFM is the point where flex duct's penalty becomes obvious. The same 8 in in smooth pipe versus compressed flex can differ enough in delivered air to change how a room actually feels.
Working the other way round? See how many CFM a 8 in duct carries, or size a real system with the calculator.