What size duct for 400 CFM?
10 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 1,497 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, 400 CFM is roughly 1.0 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.
400 CFM in each candidate size
| Round size | Velocity | Friction / 100 ft | Verdict |
|---|---|---|---|
| 7 in | 1,497 fpm | 0.489 in. w.c. | Runs fast |
| 10 in ← recommended | 733 fpm | 0.084 in. w.c. | Within range |
| 11 in | 606 fpm | 0.053 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 400 CFM
If 10 in round will not fit the cavity, these carry the same air at the same friction loss.
| Shape | Size | Aspect ratio |
|---|---|---|
| Round | 10 in | 1.0 |
| Rectangular | × in | NaN |
| Rectangular | × in | NaN |
| Flat oval | 16 × 6 in | 2.7 |
What moves 400 CFM
- One ton of cooling
- Zone trunk
- Large commercial branch
The catch at 400 CFM
400 CFM is one ton of cooling, the single most useful conversion in HVAC. Multiply system tonnage by 400 and size the trunk for that — it is the fastest sanity check on any existing duct system.
Working the other way round? See how many CFM a 10 in duct carries, or size a real system with the calculator.