Duct Size Calculator (CFM & Velocity)
Find the round duct diameter — and equivalent square duct — needed to carry a given airflow at a chosen air velocity.
How it works
Duct area = CFM ÷ velocity · Round diameter = √(4 × area ÷ π) · velocity in ft/min, area converted to in²
Airflow equals duct cross-section times air velocity, so once you pick a target velocity the required area — and from it the round diameter — follows directly. Velocity is the design lever: too high and the duct whistles and wastes fan energy on friction, too low and the duct grows bulky and expensive. Residential branch ducts are usually sized around 600–900 ft/min, with larger trunk lines run a bit faster.
Worked example
For 400 CFM at 700 ft/min, the area is 400 ÷ 700 = 0.571 ft² = 82.3 in². The round diameter is √(4 × 82.3 ÷ π) = 10.2 in, so a 10-inch duct is the practical choice (or round up to 12 in for extra quiet).
Frequently asked questions
What air velocity should I design for?
For quiet homes, aim for roughly 700 ft/min in branch runs and 700–900 ft/min in the main trunk. Commercial and high-static systems run faster, but noise and friction rise quickly above about 1,000 ft/min.
Round or rectangular duct — does it matter?
Round duct carries a given airflow with less material and lower friction than rectangular for the same area, but rectangular fits flat cavities like joist bays. This calculator gives both the round diameter and the equal-area square so you can compare.
Should I round up or down to a standard size?
Round up. A slightly larger duct lowers velocity, friction and noise, whereas undersizing raises static pressure and can starve the room of air.
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This calculator is for educational purposes. Double-check important results before acting on them.