What size duct for 800 CFM?
13 inch round — to stay quiet (900 fpm or below).
If commercial velocity is acceptable (up to 1500 fpm), 10 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, 800 CFM is roughly 2.0 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.
800 CFM in each candidate size
| Round size | Velocity | Friction / 100 ft | Verdict |
|---|---|---|---|
| 10 in | 1,467 fpm | 0.304 in. w.c. | Runs fast |
| 13 in ← recommended | 868 fpm | 0.083 in. w.c. | Within range |
| 14 in | 748 fpm | 0.058 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 800 CFM
If 13 in round will not fit the cavity, these carry the same air at the same friction loss.
| Shape | Size | Aspect ratio |
|---|---|---|
| Round | 13 in | 1.0 |
| Rectangular | × in | NaN |
| Rectangular | × in | NaN |
| Flat oval | 20 × 8 in | 2.5 |
What moves 800 CFM
- Two-ton system main
- Large residential trunk
- Commercial riser
The catch at 800 CFM
800 CFM is two tons and the most common undersizing failure in residential retrofit. Equipment gets upgraded to two tons while the original trunk stays at 10 in, and the system never delivers its rated capacity.
Working the other way round? See how many CFM a 13 in duct carries, or size a real system with the calculator.