What size duct for 600 CFM?
12 inch round — to stay quiet (900 fpm or below).
If commercial velocity is acceptable (up to 1500 fpm), 9 in will pass it — but at 1,358 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, 600 CFM is roughly 1.5 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.
600 CFM in each candidate size
| Round size | Velocity | Friction / 100 ft | Verdict |
|---|---|---|---|
| 9 in | 1,358 fpm | 0.299 in. w.c. | Runs fast |
| 12 in ← recommended | 764 fpm | 0.073 in. w.c. | Within range |
| 13 in | 651 fpm | 0.049 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 600 CFM
If 12 in round will not fit the cavity, these carry the same air at the same friction loss.
| Shape | Size | Aspect ratio |
|---|---|---|
| Round | 12 in | 1.0 |
| Rectangular | × in | NaN |
| Rectangular | × in | NaN |
| Rectangular | × in | NaN |
| Flat oval | 18 × 8 in | 2.3 |
What moves 600 CFM
- 1.5 ton system
- Residential trunk
- Multi-register branch
The catch at 600 CFM
600 CFM is 1.5 tons. It fits comfortably in 12 in but is marginal in 10 in — and 10 in is what a great many 1.5–2 ton systems were actually built with.
Working the other way round? See how many CFM a 12 in duct carries, or size a real system with the calculator.