GlenCalc

Duct size calculator

Duct size sets air speed: too small is noisy and starves rooms, too large wastes space and money. Residential designs commonly keep supply trunks around 700 to 900 feet per minute and branches around 600.

Your answer

Round duct11 in diameterExact 10.24 in
Rectangle 8 in high11 in wide
Free area needed82.3 sq in
Velocity in a 11 in round duct606 ft/min

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How we worked it out

Updated as you change the numbers.

  1. Area = CFM ÷ velocity (sq ft) × 144 for square inches. Round diameter = √(4 × area ÷ π).

Method and assumptions

This is the velocity method. Full residential design uses ACCA Manual D, which sizes ducts by friction rate and equivalent length; rectangular ducts carry a little less than a round duct of the same area because of friction in the corners.

Questions

What size duct for 400 CFM?

About 11 in round at 700 ft/min, or 8 × 11 in rectangular.

Sources

Method and figures checked on . Every worked answer on this page is covered by an automated test. Found a mistake? Tell us.

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