Air distribution · Duct sizing
Ductulator
At the standard 0.1 in/100 ft friction rate, 400 CFM needs a 10-inch round duct (or 11 × 8 rectangular). Enter your airflow and friction rate to size any run — round or rectangular — with velocity checked automatically.
Airflow & friction
- Exact diameter
- 9.8 in
- Velocity @ 10"
- 733 fpm
Round duct size
10in ⌀
exact 9.8 in @ 0.1 in/100 ft
Duct size chart
Round galvanized duct at 0.1 in. w.c./100 ft — the standard residential design point. Velocity shown at the standard size.
| Airflow (CFM) | Exact ⌀ (in) | Standard size | Velocity (fpm) |
|---|---|---|---|
| 50 | 4.5 | 5" | 367 |
| 75 | 5.2 | 6" | 382 |
| 100 | 5.8 | 6" | 509 |
| 150 | 6.8 | 7" | 561 |
| 200 | 7.6 | 8" | 573 |
| 300 | 8.8 | 9" | 679 |
| 400 | 9.8 | 10" | 733 |
| 600 | 11.5 | 12" | 764 |
| 800 | 12.8 | 14" | 748 |
| 1,000 | 13.9 | 14" | 935 |
| 1,200 | 14.9 | 16" | 859 |
| 1,600 | 16.6 | 18" | 905 |
| 2,000 | 18.1 | 20" | 917 |
Round to rectangular equivalents
Huebscher equivalent diameters for common rectangular sizes.
| Rectangular (in) | Equivalent round ⌀ |
|---|---|
| 8 × 6 | 7.6 in |
| 10 × 6 | 8.4 in |
| 10 × 8 | 9.8 in |
| 12 × 8 | 10.7 in |
| 12 × 10 | 12 in |
| 14 × 10 | 12.9 in |
| 16 × 10 | 13.7 in |
| 18 × 12 | 16 in |
| 20 × 12 | 16.8 in |
| 24 × 12 | 18.3 in |
The formulas
friction (in/100 ft) = 0.109136 × CFM^1.9 ÷ D^5.02 (galvanized, ASHRAE)
D (in) = (0.109136 × CFM^1.9 ÷ friction)^(1/5.02)
velocity (fpm) = CFM ÷ (π × (D/24)²)
De rectangular = 1.30 × (a×b)^0.625 ÷ (a+b)^0.25 (Huebscher)Method: equal friction, based on the ASHRAE Fundamentals correlations for galvanized duct. For flexible duct, size one diameter up. Fitting losses are not included — treat results as straight-run sizing.
Sizing the equipment too? Start from the BTU calculator and the CFM calculator.
FAQ
- What is a ductulator?
- A ductulator is the sizing tool HVAC installers use to pick a duct diameter from airflow (CFM) and a design friction rate. The classic version is a rotating cardboard wheel; this page does the same equal-friction calculation digitally.
- What friction rate should I use?
- 0.1 in. w.c. per 100 ft is the common default for residential supply ducts. Drop to 0.05–0.08 for long runs or noise-sensitive spaces, and only go higher for short branches. Flexible duct has more resistance than sheet metal — when using flex, size one diameter up or design at a lower friction rate.
- How many CFM does a 6 inch duct handle?
- At the standard 0.1 in/100 ft design friction, a 6-inch round metal duct carries roughly 100–110 CFM at about 550 fpm. At 0.05 in/100 ft it drops to about 75 CFM.
- How do I convert a round duct to rectangular?
- Use the Huebscher equivalent-diameter formula: De = 1.30 × (a×b)^0.625 ÷ (a+b)^0.25. A 10 × 6 in rectangular duct, for example, is equivalent to an 8.4 in round duct — not to the 8.7 in circle of the same area, because the corners add friction.
- What air velocity is acceptable in ducts?
- Keep residential supply trunks near 700–900 fpm and branches near 600 fpm to stay quiet; returns run slightly lower. Commercial systems tolerate 1,200–1,800 fpm in mains where noise matters less.
- How many CFM per ton of cooling?
- Plan around 400 CFM per ton of air conditioning (350 in humid climates, up to 450 in dry ones). A 3-ton system therefore needs about 1,200 CFM — a 16-inch round trunk at standard friction.