Air distribution · Duct sizing
What duct size for 400 CFM?
400 CFM needs a 10-inch round duct at the standard 0.10 in/100 ft friction rate — the exact diameter works out to 9.8 in, at 733 fpm. For long or quiet runs, step up to 12 inches.
Size by design friction rate
| Friction (in/100 ft) | Exact ⌀ | Standard size | Velocity (fpm) |
|---|---|---|---|
| 0.05 | 11.3 in | 12 in | 509 |
| 0.08 | 10.3 in | 12 in | 509 |
| 0.10 | 9.8 in | 10 in | 733 |
| 0.15 | 9.1 in | 10 in | 733 |
Rectangular options
Rectangular ducts equivalent to the 9.8-inch round requirement. Only buildable shapes are listed — nothing narrower than 4 inches or beyond a 4:1 aspect ratio.
| Duct size | Equivalent ⌀ |
|---|---|
| 15 × 6 in | 10.1 in |
| 11 × 8 in | 10.2 in |
| 10 × 9 in | 10.4 in |
| 12 × 7 in | 9.9 in |
FAQ
- What size duct do I need for 400 CFM?
- A 10-inch round duct at the standard 0.10 in. w.c. per 100 ft friction rate — the exact calculated diameter is 9.8 in, and the next standard size up is 10 in. Air moves through it at 733 fpm.
- What if the run is long or noise matters?
- Design at a lower friction rate. At 0.05 in/100 ft the same 400 CFM needs a 12-inch duct — larger, but quieter and with less pressure loss over a long run.
- How many tons of cooling is 400 CFM?
- At the standard 400 CFM per ton, 400 CFM corresponds to about 1 tons. Humid climates design closer to 350 CFM per ton, dry climates up to 450.
Other airflows: 300 CFM · 500 CFM · any airflow · standard sizes