Duct Size Calculator (Round and Rectangular)
Enter the airflow and a friction rate (0.08 in. wc per 100 ft is the usual residential design value) to get the round duct diameter, the air velocity, and the equivalent rectangular size for the height you have available.
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The equations
Round duct diameter from the equal friction method: D = (0.109136 × CFM1.9 ÷ Δp)0.199, where Δp is in inches of water per 100 feet. Velocity = CFM ÷ area in ft².
The equivalent rectangular duct is the one with the same friction at the same airflow, from De = 1.30 × (a × b)0.625 ÷ (a + b)0.25. Aspect ratios above about 3:1 are inefficient. Typical residential velocities are 600 to 900 fpm on branches and up to 1,000 fpm on trunks. Final design should follow ACCA Manual D.
Worked example
400 CFM at 0.08 in. wc per 100 ft calls for a 10.3-inch round duct (about 695 fpm), so you would use an 11 in. round, or an 8 in. high rectangle about 11 in. wide.
Frequently asked questions
What friction rate should I use?
0.08 to 0.10 in. wc per 100 ft is typical for residential. Use lower for quiet applications and when you have a long run.
Do fittings count?
This sizes straight duct. Fittings add equivalent length, which Manual D accounts for when choosing your friction rate.
Related calculators
This calculator gives an estimate for planning and is not tax, legal or engineering advice. Confirm code-related results against the code edition adopted in your jurisdiction and with your authority having jurisdiction, and follow the manufacturer's instructions.