Reference · 0.08 in.wg/100ft · Galvanized
CFM to duct size chart
Round, rectangular (≈2:1 Huebscher equivalent), and taut-flex sizes at the standard residential friction rate — with actual velocity and metric. For exact inputs use the calculator.
| CFM | Round | Rect (≈2:1) | Velocity | Flex (taut) | Metric |
|---|---|---|---|---|---|
| 100 | 6″ | 8×6 | ~510 FPM | 5″ | 2.1 Pa/m · 47 L/s |
| 150 | 7″ | 10×6 | ~560 FPM | 8″ | 2.6 Pa/m · 71 L/s |
| 200 | 8″ | 10×8 | ~575 FPM | 9″ | 2.4 Pa/m · 94 L/s |
| 300 | 9″ | 12×8 | ~680 FPM | 10″ | 2.9 Pa/m · 142 L/s |
| 400 | 12″ | 14×8 | ~510 FPM | 12″ | 1.7 Pa/m · 189 L/s |
| 500 | 12″ | 14×10 | ~635 FPM | 14″ | 2.3 Pa/m · 236 L/s |
| 600 | 14″ | 16×10 | ~560 FPM | 14″ | 1.9 Pa/m · 283 L/s |
| 800 | 14″ | 18×10 | ~750 FPM | 16″ | 2.8 Pa/m · 378 L/s |
| 1000 | 16″ | 20×10 | ~715 FPM | 18″ | 2.4 Pa/m · 472 L/s |
| 1200 | 16″ | 22×10 | ~860 FPM | 18″ | 3.1 Pa/m · 566 L/s |
| 1500 | 18″ | 24×12 | ~850 FPM | 20″ | 2.8 Pa/m · 708 L/s |
| 2000 | 20″ | 28×12 | ~920 FPM | 22″ | 3.2 Pa/m · 944 L/s |
How to read this chart
Sizes assume galvanized steel at sea level and 0.08 in.wg/100ft. Velocity is the actual speed in the listed standard size (rounding up always drops velocity below target — the safe direction). For quiet bedrooms stay under ~600 FPM; size up one diameter if a row lands hot. At 0.06, go one size up across the board; at 0.10, most rows drop one size but run louder.
Friction-rate adjustments
Long runs and quiet zones (0.06): 400 CFM → 12″ still, but 800 CFM → 16″. Short trunks (0.10): 400 CFM → 10″, 1200 CFM → 14–16″. Above 5,000 ft altitude, treat every row as one size more conservative or enter altitude in the calculator. Compressed flex is off-chart — fix the install, not the size.