Air distribution · Duct sizing
What duct size for 4,000 CFM?
4,000 CFM needs a 24-inch round duct at the standard 0.10 in/100 ft friction rate — the exact diameter works out to 23.5 in, at 1,273 fpm. For long or quiet runs, step up to 28 inches.
Size by design friction rate
| Friction (in/100 ft) | Exact ⌀ | Standard size | Velocity (fpm) |
|---|---|---|---|
| 0.05 | 27 in | 28 in | 935 |
| 0.08 | 24.6 in | 26 in | 1,085 |
| 0.10 | 23.5 in | 24 in | 1,273 |
| 0.15 | 21.7 in | 22 in | 1,515 |
Rectangular options
Rectangular ducts equivalent to the 23.5-inch round requirement. Only buildable shapes are listed — nothing narrower than 4 inches or beyond a 4:1 aspect ratio.
| Duct size | Equivalent ⌀ |
|---|---|
| 43 × 12 in | 23.7 in |
FAQ
- What size duct do I need for 4,000 CFM?
- A 24-inch round duct at the standard 0.10 in. w.c. per 100 ft friction rate — the exact calculated diameter is 23.5 in, and the next standard size up is 24 in. Air moves through it at 1,273 fpm.
- What if the run is long or noise matters?
- Design at a lower friction rate. At 0.05 in/100 ft the same 4,000 CFM needs a 28-inch duct — larger, but quieter and with less pressure loss over a long run.
- How many tons of cooling is 4,000 CFM?
- At the standard 400 CFM per ton, 4,000 CFM corresponds to about 10 tons. Humid climates design closer to 350 CFM per ton, dry climates up to 450.
Other airflows: 3,000 CFM · any airflow · standard sizes