Duct Size Calculator
Calculate the correct duct diameter for a given CFM and velocity. Includes equivalent rectangular duct sizing.
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CFM
Cubic feet per minute — check your Manual D calculation or equipment specs
Supply ducts carry conditioned air to rooms. Return ducts bring air back to the unit.
FPM
Feet per minute. 700 FPM is standard for supply, 500 FPM for return. Higher = more noise.
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