What size duct for 200 CFM?
7 inch round — to stay quiet (900 fpm or below).
If commercial velocity is acceptable (up to 1500 fpm), 5 in will pass it — but at 1,467 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, 200 CFM is roughly 0.50 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.
200 CFM in each candidate size
| Round size | Velocity | Friction / 100 ft | Verdict |
|---|---|---|---|
| 5 in | 1,467 fpm | 0.714 in. w.c. | Runs fast |
| 7 in ← recommended | 748 fpm | 0.136 in. w.c. | Within range |
| 8 in | 573 fpm | 0.071 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 200 CFM
If 7 in round will not fit the cavity, these carry the same air at the same friction loss.
| Shape | Size | Aspect ratio |
|---|---|---|
| Round | 7 in | 1.0 |
| Rectangular | × in | NaN |
| Flat oval | 8 × 6 in | 1.3 |
What moves 200 CFM
- Living-room supply
- Medium range hood
- Two combined bedroom runs
The catch at 200 CFM
200 CFM is roughly what a single ton of cooling needs at the usual 400 CFM/ton rule of thumb — a useful anchor: this is a half-ton of capacity down one duct.
Working the other way round? See how many CFM a 7 in duct carries, or size a real system with the calculator.