What size duct for 1,000 CFM?
16 inch round — to stay quiet (900 fpm or below).
If commercial velocity is acceptable (up to 1500 fpm), 12 in will pass it — but at 1,273 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, 1,000 CFM is roughly 2.5 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.
1,000 CFM in each candidate size
| Round size | Velocity | Friction / 100 ft | Verdict |
|---|---|---|---|
| 12 in | 1,273 fpm | 0.187 in. w.c. | Runs fast |
| 16 in ← recommended | 716 fpm | 0.045 in. w.c. | Within range |
| 18 in | 566 fpm | 0.026 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 1,000 CFM
If 16 in round will not fit the cavity, these carry the same air at the same friction loss.
| Shape | Size | Aspect ratio |
|---|---|---|
| Round | 16 in | 1.0 |
| Rectangular | × in | NaN |
| Rectangular | × in | NaN |
| Rectangular | × in | NaN |
| Flat oval | 24 × 10 in | 2.4 |
What moves 1,000 CFM
- 2.5 ton system
- Large trunk
- Commercial main
The catch at 1,000 CFM
1,000 CFM needs 14 in to stay in the residential quiet band, but only 12 in if commercial velocities are acceptable. Which band applies is a comfort decision, not a code one — and it changes the answer by a full size.
Working the other way round? See how many CFM a 16 in duct carries, or size a real system with the calculator.