Air distribution · Duct sizing
What duct size for 800 CFM?
800 CFM needs a 14-inch round duct at the standard 0.10 in/100 ft friction rate — the exact diameter works out to 12.8 in, at 748 fpm. For long or quiet runs, step up to 16 inches.
Size by design friction rate
| Friction (in/100 ft) | Exact ⌀ | Standard size | Velocity (fpm) |
|---|---|---|---|
| 0.05 | 14.7 in | 16 in | 573 |
| 0.08 | 13.4 in | 14 in | 748 |
| 0.10 | 12.8 in | 14 in | 748 |
| 0.15 | 11.8 in | 12 in | 1,019 |
Rectangular options
Rectangular ducts equivalent to the 12.8-inch round requirement. Only buildable shapes are listed — nothing narrower than 4 inches or beyond a 4:1 aspect ratio.
| Duct size | Equivalent ⌀ |
|---|---|
| 18 × 8 in | 12.9 in |
| 14 × 10 in | 12.9 in |
| 12 × 12 in | 13.1 in |
FAQ
- What size duct do I need for 800 CFM?
- A 14-inch round duct at the standard 0.10 in. w.c. per 100 ft friction rate — the exact calculated diameter is 12.8 in, and the next standard size up is 14 in. Air moves through it at 748 fpm.
- What if the run is long or noise matters?
- Design at a lower friction rate. At 0.05 in/100 ft the same 800 CFM needs a 16-inch duct — larger, but quieter and with less pressure loss over a long run.
- How many tons of cooling is 800 CFM?
- At the standard 400 CFM per ton, 800 CFM corresponds to about 2 tons. Humid climates design closer to 350 CFM per ton, dry climates up to 450.
Other airflows: 600 CFM · 1,000 CFM · any airflow · standard sizes