Air distribution · Duct sizing
What duct size for 200 CFM?
200 CFM needs a 8-inch round duct at the standard 0.10 in/100 ft friction rate — the exact diameter works out to 7.6 in, at 573 fpm. For long or quiet runs, step up to 9 inches.
Size by design friction rate
| Friction (in/100 ft) | Exact ⌀ | Standard size | Velocity (fpm) |
|---|---|---|---|
| 0.05 | 8.7 in | 9 in | 453 |
| 0.08 | 7.9 in | 8 in | 573 |
| 0.10 | 7.6 in | 8 in | 573 |
| 0.15 | 7 in | 7 in | 748 |
Rectangular options
Rectangular ducts equivalent to the 7.6-inch round requirement. Only buildable shapes are listed — nothing narrower than 4 inches or beyond a 4:1 aspect ratio.
| Duct size | Equivalent ⌀ |
|---|---|
| 9 × 6 in | 8 in |
| 8 × 7 in | 8.2 in |
| 10 × 5 in | 7.6 in |
| 12 × 5 in | 8.3 in |
FAQ
- What size duct do I need for 200 CFM?
- A 8-inch round duct at the standard 0.10 in. w.c. per 100 ft friction rate — the exact calculated diameter is 7.6 in, and the next standard size up is 8 in. Air moves through it at 573 fpm.
- What if the run is long or noise matters?
- Design at a lower friction rate. At 0.05 in/100 ft the same 200 CFM needs a 9-inch duct — larger, but quieter and with less pressure loss over a long run.
- How many tons of cooling is 200 CFM?
- At the standard 400 CFM per ton, 200 CFM corresponds to about 0.5 tons. Humid climates design closer to 350 CFM per ton, dry climates up to 450.
Other airflows: 150 CFM · 300 CFM · any airflow · standard sizes