Air distribution · Duct sizing
What duct size for 1,200 CFM?
1,200 CFM needs a 16-inch round duct at the standard 0.10 in/100 ft friction rate — the exact diameter works out to 14.9 in, at 859 fpm. For long or quiet runs, step up to 18 inches.
Size by design friction rate
| Friction (in/100 ft) | Exact ⌀ | Standard size | Velocity (fpm) |
|---|---|---|---|
| 0.05 | 17.1 in | 18 in | 679 |
| 0.08 | 15.6 in | 16 in | 859 |
| 0.10 | 14.9 in | 16 in | 859 |
| 0.15 | 13.7 in | 14 in | 1,123 |
Rectangular options
Rectangular ducts equivalent to the 14.9-inch round requirement. Only buildable shapes are listed — nothing narrower than 4 inches or beyond a 4:1 aspect ratio.
| Duct size | Equivalent ⌀ |
|---|---|
| 26 × 8 in | 15.1 in |
| 20 × 10 in | 15.2 in |
| 16 × 12 in | 15.1 in |
FAQ
- What size duct do I need for 1,200 CFM?
- A 16-inch round duct at the standard 0.10 in. w.c. per 100 ft friction rate — the exact calculated diameter is 14.9 in, and the next standard size up is 16 in. Air moves through it at 859 fpm.
- What if the run is long or noise matters?
- Design at a lower friction rate. At 0.05 in/100 ft the same 1,200 CFM needs a 18-inch duct — larger, but quieter and with less pressure loss over a long run.
- How many tons of cooling is 1,200 CFM?
- At the standard 400 CFM per ton, 1,200 CFM corresponds to about 3 tons. Humid climates design closer to 350 CFM per ton, dry climates up to 450.
Other airflows: 1,000 CFM · 1,400 CFM · any airflow · standard sizes