HVAC
Duct Size Calculator
Size a duct for a given airflow and design velocity, in round and rectangular equivalents.
Result
This is velocity-based sizing, which is a first approximation. Proper duct design (ACCA Manual D) works from a total friction rate across the whole system including fittings, and equivalent rectangular sizing is not simply equal area. It carries a friction penalty.
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About this calculator
What velocity should I design for?
Six hundred to 900 fpm for branch runs and 900–1,200 for trunks in residential work. Higher velocities generate noise, which is the most common complaint on undersized ducts.
Is a rectangular duct of equal area the same as round?
No. Rectangular ducts have more surface area per unit of flow and therefore more friction, particularly as the aspect ratio increases. Keep them as close to square as space allows.
The formula
duct area = CFM ÷ velocity; round diameter = √(4 × area ÷ π)
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