Duct Size Calculator

Find the round or rectangular duct size for an airflow and friction rate, or check the friction and velocity of a duct you have.

What Do You Want to Find?
0.08 to 0.10 is typical for homes.
To size a rectangular duct of this height with the same friction.
Fractions and decimals both work.
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Inch Round Duct
Exact diameter
Velocity
Friction
Rectangular option
Area
Next Standard Size

Duct Schedule

Items you add are saved in this browser.

RunCFMSizeFPMIn./100 Ft.Remove

How to Size a Duct

The equal friction method picks a pressure loss per 100 feet of duct and sizes every run to it. The standard duct friction chart for galvanized steel duct can be written as a formula, which this page solves for the diameter.

Friction (in. w.c. per 100 ft) = 0.109136 × CFM1.9 ÷ D5.02
Velocity (ft/min) = CFM ÷ area (ft²)   Rectangular equivalent: De = 1.30 × (a × b)0.625 ÷ (a + b)0.25

The rectangular equivalent (Huebscher's formula) gives the round diameter with the same friction and airflow as an a × b rectangular duct.

Worked Example

Round Duct Capacity at 0.08 In. per 100 Ft.

DiameterCFMVelocity (FPM)

Tips

The formula fits smooth galvanized sheet metal duct. Flex duct has far more friction, often double or more when it's compressed or sags, so size it larger. Residential supply branches are usually kept under about 700 to 900 FPM to stay quiet. For a full design, ACCA Manual D ties duct sizes to the blower's available static pressure.