What size duct for 2,000 CFM?
22 inch round — to stay quiet (900 fpm or below).
If commercial velocity is acceptable (up to 1500 fpm), 16 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, 2,000 CFM is roughly 5.0 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.
2,000 CFM in each candidate size
| Round size | Velocity | Friction / 100 ft | Verdict |
|---|---|---|---|
| 16 in | 1,432 fpm | 0.164 in. w.c. | Runs fast |
| 22 in ← recommended | 758 fpm | 0.034 in. w.c. | Within range |
| 24 in | 637 fpm | 0.022 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 2,000 CFM
If 22 in round will not fit the cavity, these carry the same air at the same friction loss.
| Shape | Size | Aspect ratio |
|---|---|---|
| Round | 22 in | 1.0 |
| Rectangular | × in | NaN |
| Rectangular | × in | NaN |
| Rectangular | × in | NaN |
| Flat oval | 38 × 12 in | 3.2 |
What moves 2,000 CFM
- Five-ton system main
- Commercial rooftop connection
- Large multi-zone main
The catch at 2,000 CFM
2,000 CFM is five tons, the top of typical residential equipment. At this volume a single sharp elbow can cost more static pressure than 20 ft of straight duct, so fitting count matters more than duct length.
Working the other way round? See how many CFM a 22 in duct carries, or size a real system with the calculator.