Round Duct Size Calculator (Velocity Method)

Pick a velocity suited to the duct's role (supply branch, trunk, return) and solve for diameter.

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Formula

D (in) = √(4 × 144 × CFM ÷ (π × fpm))

Example calculation

400 CFM at 700 fpm → 10.2 in → use 11 in or verify 10 in friction.

Practical HVAC use

Quick sizing before a friction-rate check.

Common mistakes

Ignoring friction rate and available static pressure.

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Calculate air velocity in FPM and m/s in round or rectangular ducts from airflow and duct size.

Educational tool. Results are estimates from the stated formulas; always confirm with manufacturer data and the code adopted in your jurisdiction. All calculators