How many CFM can a 10 inch duct carry?
About 491 CFM — at the top of the quiet residential range (900 fpm).
A 10 in round duct has 0.545 ft² of free area. Pushed to the commercial velocity cap (1500 fpm) it will pass roughly 818 CFM, but that is loud in a home. There is no single "max CFM" for a duct — only the airflow at which it gets too noisy or too restrictive, which is what these numbers describe.
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.
10 inch duct: airflow, velocity and friction rate
| Airflow | Velocity | Friction / 100 ft | Verdict |
|---|---|---|---|
| 125 CFM | 229 fpm | 0.010 in. w.c. | Within range |
| 245 CFM | 449 fpm | 0.035 in. w.c. | Within range |
| 370 CFM | 678 fpm | 0.073 in. w.c. | Within range |
| 490 CFM | 898 fpm | 0.122 in. w.c. | Within range |
| 615 CFM | 1,128 fpm | 0.186 in. w.c. | Runs fast |
| 735 CFM | 1,348 fpm | 0.259 in. w.c. | Runs fast |
Velocity V = 183.3 × CFM / D². Friction from Darcy-Weisbach with the Haaland factor, clean galvanized (ε = 0.0003 ft). Residential systems are usually designed near 0.1 in. w.c. per 100 ft.
Rectangular and oval equivalents of 10 inch round
These carry the same air at the same friction loss — the equal-friction equivalents, not equal area. A rectangle always needs more cross-section than the round duct it replaces.
| Shape | Size | Aspect ratio |
|---|---|---|
| Round | 10 in | 1.0 |
| Rectangular | × in | NaN |
| Rectangular | × in | NaN |
| Flat oval | 16 × 6 in | 2.7 |
Huebscher (rectangular) and Heyt & Diaz (flat oval). Keep aspect ratio under about 4:1 — past that, friction rises faster than the equivalence relation predicts.
What 10 inch duct is used for
- Small trunk lines
- Multiple-register branches
- Commercial branch runs
The 10 inch mistake worth knowing
10 in is the smallest size normally used as a trunk, and it is where the friction rate starts mattering more than the velocity. On long runs the pressure loss decides the size well before the noise does.