What size duct for 100 CFM?
5 inch round — to stay quiet (900 fpm or below).
If commercial velocity is acceptable (up to 1500 fpm), 4 in will pass it — but at 1,146 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.
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.
100 CFM in each candidate size
| Round size | Velocity | Friction / 100 ft | Verdict |
|---|---|---|---|
| 4 in | 1,146 fpm | 0.597 in. w.c. | Runs fast |
| 5 in ← recommended | 733 fpm | 0.199 in. w.c. | Within range |
| 6 in | 509 fpm | 0.082 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 100 CFM
If 5 in round will not fit the cavity, these carry the same air at the same friction loss.
| Shape | Size | Aspect ratio |
|---|---|---|
| Round | 5 in | 1.0 |
| Flat oval | 8 × 7 in | 1.1 |
What moves 100 CFM
- Large bath fan
- Bedroom supply register
- Small room heating or cooling
The catch at 100 CFM
100 CFM is the number most people put on 4 in duct and should not. It needs 5 in to stay in the quiet band; on 4 in it runs near 1,150 fpm, which is audible in a bedroom.
Working the other way round? See how many CFM a 5 in duct carries, or size a real system with the calculator.