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.
Looking for a specific size? the duct size chart lists airflow by diameter and diameter by airflow, alongside standard duct sizes or return air 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.