Duct Sizing Calculator
Size ducts using equal friction or velocity method.
CFM Capacity by Duct Size
@ 0.08 in. w.g./100 ft friction rate
| Round | CFM | Velocity (FPM) |
|---|---|---|
| 4" | 35 | 400 |
| 5" | 65 | 475 |
| 6" | 100 | 510 |
| 7" | 150 | 560 |
| 8" | 210 | 600 |
| 10" | 370 | 680 |
| 12" | 580 | 740 |
| 14" | 850 | 795 |
| 16" | 1200 | 860 |
| 18" | 1600 | 905 |
| 20" | 2100 | 965 |
Maximum Velocities (FPM)
| Application | Residential | Commercial |
|---|---|---|
| Main Supply | 700-900 | 1000-1300 |
| Branch Supply | 600-700 | 800-1000 |
| Return | 500-600 | 800-1000 |