How many CFM can a 20 inch duct carry?
About 1,963 CFM — at the top of the quiet residential range (900 fpm).
A 20 in round duct has 2.182 ft² of free area. Pushed to the commercial velocity cap (1500 fpm) it will pass roughly 3,272 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.
20 inch duct: airflow, velocity and friction rate
| Airflow | Velocity | Friction / 100 ft | Verdict |
|---|---|---|---|
| 490 CFM | 225 fpm | 0.004 in. w.c. | Within range |
| 980 CFM | 449 fpm | 0.015 in. w.c. | Within range |
| 1,475 CFM | 676 fpm | 0.031 in. w.c. | Within range |
| 1,965 CFM | 901 fpm | 0.053 in. w.c. | Runs fast |
| 2,455 CFM | 1,125 fpm | 0.080 in. w.c. | Runs fast |
| 2,945 CFM | 1,350 fpm | 0.112 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 20 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 | 20 in | 1.0 |
| Rectangular | × in | NaN |
| Rectangular | × in | NaN |
| Rectangular | × in | NaN |
| Flat oval | 32 × 12 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 20 inch duct is used for
- Commercial mains
- Large multi-zone trunks
- Rooftop unit connections
The 20 inch mistake worth knowing
20 in is where round duct starts losing to rectangular on installed cost — not because it performs worse, but because it stops fitting in standard joist and ceiling cavities. This is usually the point where a design converts to rectangular or flat oval.