Reference chart
Duct Size Chart
Round duct sizes for 50–2,000 CFM at three friction rates, computed with the same equal-friction method as our ductulator. Standard nominal sizes, galvanized sheet metal.
| CFM | @ 0.06 in/100 ft | @ 0.08 in/100 ft | @ 0.10 in/100 ft |
|---|---|---|---|
| 50 | 5" | 5" | 5" |
| 75 | 6" | 6" | 6" |
| 100 | 7" | 7" | 6" |
| 150 | 8" | 8" | 7" |
| 200 | 9" | 8" | 8" |
| 300 | 10" | 10" | 9" |
| 400 | 12" | 12" | 10" |
| 500 | 12" | 12" | 12" |
| 600 | 14" | 12" | 12" |
| 800 | 16" | 14" | 14" |
| 1,000 | 16" | 16" | 14" |
| 1,200 | 18" | 16" | 16" |
| 1,400 | 18" | 18" | 16" |
| 1,600 | 20" | 18" | 18" |
| 2,000 | 22" | 20" | 20" |
| 2,500 | 22" | 22" | 20" |
| 3,000 | 24" | 24" | 22" |
| 4,000 | 28" | 26" | 24" |
Rectangular duct equivalents
Huebscher equivalent round diameter for common rectangular sizes.
| Rectangular (in) | Equivalent round ⌀ |
|---|---|
| 8 × 4 | 6.1 in |
| 8 × 6 | 7.6 in |
| 10 × 6 | 8.4 in |
| 10 × 8 | 9.8 in |
| 12 × 6 | 9.1 in |
| 12 × 8 | 10.7 in |
| 12 × 10 | 12 in |
| 14 × 8 | 11.5 in |
| 14 × 10 | 12.9 in |
| 16 × 8 | 12.2 in |
| 16 × 10 | 13.7 in |
| 18 × 10 | 14.5 in |
| 18 × 12 | 16 in |
| 20 × 10 | 15.2 in |
| 20 × 12 | 16.8 in |
| 24 × 12 | 18.3 in |
| 30 × 12 | 20.2 in |
How many CFM does each duct size carry?
The same relationship read the other way: airflow a round duct carries at four design friction rates, with velocity at 0.10 as the noise check. Keep branch velocity under about 900 fpm in living spaces.
| Diameter | @ 0.05 | @ 0.08 | @ 0.10 | @ 0.15 | fpm @ 0.10 |
|---|---|---|---|---|---|
| 4" | 26 | 33 | 37 | 46 | 426 |
| 5" | 47 | 60 | 67 | 83 | 492 |
| 6" | 75 | 97 | 109 | 134 | 553 |
| 7" | 113 | 145 | 163 | 202 | 611 |
| 8" | 161 | 207 | 232 | 288 | 666 |
| 9" | 220 | 282 | 317 | 393 | 718 |
| 10" | 291 | 372 | 419 | 519 | 768 |
| 12" | 471 | 603 | 678 | 839 | 863 |
| 14" | 708 | 906 | 1,019 | 1,262 | 953 |
| 16" | 1,007 | 1,290 | 1,450 | 1,795 | 1,039 |
| 18" | 1,374 | 1,760 | 1,980 | 2,451 | 1,120 |
| 20" | 1,816 | 2,325 | 2,615 | 3,237 | 1,199 |
Friction rates are in inches of water column per 100 ft. Figures assume galvanized sheet metal — for flexible duct, go one diameter up. See also standard duct sizes and return air duct sizing.
FAQ
- What friction rate does this chart use?
- Three common design points: 0.10 in/100 ft (standard residential), 0.08 (typical trunk), and 0.06 (long runs or noise-sensitive design). Lower friction means bigger, quieter ducts for the same airflow.
- Are these sizes for flex duct?
- No — they assume galvanized sheet metal. Flexible duct has noticeably more resistance: go one diameter up, keep it stretched tight, and avoid sharp bends.
- How do I read the rectangular table?
- Find the round diameter the airflow needs, then pick any rectangular size whose equivalent diameter matches or exceeds it. Equivalents follow the Huebscher formula, which accounts for the extra friction of corners.