Duct Size Calculator — CFM to Round & Rectangular Duct
Enter the airflow and your target velocity or friction rate. The calculator returns the round duct diameter, the equivalent rectangular size and the resulting velocity. Duct area from airflow and velocity is exact; the friction figure is the standard ASHRAE duct-friction approximation and is labelled as a reference rather than a chart lookup.
| Round duct | Area (sq ft) | CFM at 600 FPM | CFM at 900 FPM |
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How this is worked out. Required area A = CFM ÷ velocity. Round diameter = √(4A/π). Rectangular sizes are equivalent-area conversions at a 2:1 aspect ratio. Friction is estimated with the ASHRAE duct-friction approximation for galvanised steel and is a reference figure, not a substituted chart value. Confirm against the equipment schedule and local code before ordering ductwork.