Duct Size Chart — CFM to Duct Size
A printable duct size chart from 100 to 10,000 CFM. Each row gives the exact required diameter, the next standard round size, a rectangular equivalent and the velocity that results — computed with Darcy-Weisbach and the Altshul-Tsal friction factor, not read off a nomogram.
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Chart Basis
Standard air at sea level, 70 °F.
CFM to Duct Size
| Airflow | Required Ø | Standard round | Rectangular | Velocity |
|---|---|---|---|---|
| 100 CFM | 6.1 in | 7 in | 10" × 5" | 374 fpm |
| 150 CFM | 7.1 in | 8 in | 12" × 6" | 430 fpm |
| 200 CFM | 7.8 in | 8 in | 12" × 6" | 573 fpm |
| 250 CFM | 8.5 in | 9 in | 12" × 6" | 566 fpm |
| 300 CFM | 9.1 in | 10 in | 14" × 7" | 550 fpm |
| 400 CFM | 10.2 in | 11 in | 16" × 8" | 606 fpm |
| 500 CFM | 11.0 in | 12 in | 16" × 8" | 637 fpm |
| 600 CFM | 11.8 in | 12 in | 16" × 8" | 764 fpm |
| 800 CFM | 13.1 in | 14 in | 20" × 10" | 748 fpm |
| 1,000 CFM | 14.3 in | 16 in | 22" × 11" | 716 fpm |
| 1,250 CFM | 15.5 in | 16 in | 22" × 11" | 895 fpm |
| 1,500 CFM | 16.6 in | 18 in | 24" × 12" | 849 fpm |
| 2,000 CFM | 18.5 in | 20 in | 28" × 14" | 917 fpm |
| 2,500 CFM | 20.2 in | 22 in | 30" × 15" | 947 fpm |
| 3,000 CFM | 21.6 in | 22 in | 30" × 15" | 1,136 fpm |
| 4,000 CFM | 24.1 in | 26 in | 36" × 18" | 1,085 fpm |
| 5,000 CFM | 26.2 in | 28 in | 38" × 19" | 1,169 fpm |
| 6,000 CFM | 28.1 in | 30 in | 40" × 20" | 1,222 fpm |
| 8,000 CFM | 31.3 in | 32 in | 44" × 22" | 1,432 fpm |
| 10,000 CFM | 34.0 in | 36 in | 48" × 24" | 1,415 fpm |
Sizes are the next standard round up from the exact requirement, so the actual velocity shown is slightly lower than the design target.
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Important Disclaimer
This calculator is for preliminary design guidance only. Final duct sizing, construction, and installation must be performed by qualified professionals and comply with applicable codes and standards.