Air distribution · Duct sizing
What duct size for 2,500 CFM?
2,500 CFM needs a 20-inch round duct at the standard 0.10 in/100 ft friction rate — the exact diameter works out to 19.7 in, at 1,146 fpm. For long or quiet runs, step up to 24 inches.
Size by design friction rate
| Friction (in/100 ft) | Exact ⌀ | Standard size | Velocity (fpm) |
|---|---|---|---|
| 0.05 | 22.6 in | 24 in | 796 |
| 0.08 | 20.6 in | 22 in | 947 |
| 0.10 | 19.7 in | 20 in | 1,146 |
| 0.15 | 18.1 in | 20 in | 1,146 |
Rectangular options
Rectangular ducts equivalent to the 19.7-inch round requirement. Only buildable shapes are listed — nothing narrower than 4 inches or beyond a 4:1 aspect ratio.
| Duct size | Equivalent ⌀ |
|---|---|
| 36 × 10 in | 19.8 in |
| 29 × 12 in | 19.9 in |
FAQ
- What size duct do I need for 2,500 CFM?
- A 20-inch round duct at the standard 0.10 in. w.c. per 100 ft friction rate — the exact calculated diameter is 19.7 in, and the next standard size up is 20 in. Air moves through it at 1,146 fpm.
- What if the run is long or noise matters?
- Design at a lower friction rate. At 0.05 in/100 ft the same 2,500 CFM needs a 24-inch duct — larger, but quieter and with less pressure loss over a long run.
- How many tons of cooling is 2,500 CFM?
- At the standard 400 CFM per ton, 2,500 CFM corresponds to about 6.25 tons. Humid climates design closer to 350 CFM per ton, dry climates up to 450.
Other airflows: 2,000 CFM · 3,000 CFM · any airflow · standard sizes