What size duct for 500 CFM?
11 inch round — to stay quiet (900 fpm or below).
If commercial velocity is acceptable (up to 1500 fpm), 8 in will pass it — but at 1,432 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, 500 CFM is roughly 1.3 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.
500 CFM in each candidate size
| Round size | Velocity | Friction / 100 ft | Verdict |
|---|---|---|---|
| 8 in | 1,432 fpm | 0.382 in. w.c. | Runs fast |
| 11 in ← recommended | 758 fpm | 0.080 in. w.c. | Within range |
| 12 in | 637 fpm | 0.052 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 500 CFM
If 11 in round will not fit the cavity, these carry the same air at the same friction loss.
| Shape | Size | Aspect ratio |
|---|---|---|
| Round | 11 in | 1.0 |
| Rectangular | × in | NaN |
| Rectangular | × in | NaN |
| Rectangular | × in | NaN |
| Flat oval | 16 × 7 in | 2.3 |
What moves 500 CFM
- Small system main
- Large zone trunk
- Commercial branch
The catch at 500 CFM
500 CFM is where the friction rate, not the velocity, starts choosing the size. A duct that passes the noise check can still starve the system if the total effective length is long.
Working the other way round? See how many CFM a 11 in duct carries, or size a real system with the calculator.