Air distribution · Duct sizing
What duct size for 100 CFM?
100 CFM needs a 6-inch round duct at the standard 0.10 in/100 ft friction rate — the exact diameter works out to 5.8 in, at 509 fpm. For long or quiet runs, step up to 7 inches.
Size by design friction rate
| Friction (in/100 ft) | Exact ⌀ | Standard size | Velocity (fpm) |
|---|---|---|---|
| 0.05 | 6.7 in | 7 in | 374 |
| 0.08 | 6.1 in | 7 in | 374 |
| 0.10 | 5.8 in | 6 in | 509 |
| 0.15 | 5.4 in | 6 in | 509 |
Rectangular options
Rectangular ducts equivalent to the 5.8-inch round requirement. Only buildable shapes are listed — nothing narrower than 4 inches or beyond a 4:1 aspect ratio.
| Duct size | Equivalent ⌀ |
|---|---|
| 6 × 5 in | 6 in |
| 8 × 4 in | 6.1 in |
| 10 × 4 in | 6.7 in |
FAQ
- What size duct do I need for 100 CFM?
- A 6-inch round duct at the standard 0.10 in. w.c. per 100 ft friction rate — the exact calculated diameter is 5.8 in, and the next standard size up is 6 in. Air moves through it at 509 fpm.
- What if the run is long or noise matters?
- Design at a lower friction rate. At 0.05 in/100 ft the same 100 CFM needs a 7-inch duct — larger, but quieter and with less pressure loss over a long run.
- How many tons of cooling is 100 CFM?
- At the standard 400 CFM per ton, 100 CFM corresponds to about 0.25 tons. Humid climates design closer to 350 CFM per ton, dry climates up to 450.
Other airflows: 150 CFM · any airflow · standard sizes