About this calculator This calculator works in three directions for round ducts: solve for CFM from diameter and velocity, solve for the minimum diameter that keeps velocity within a target limit for a given CFM, or solve for velocity from CFM and diameter. Pick a mode with the "Solve for" selector and enter the two known values — the third is computed live. Use it during layout to check airflow capacity at measured duct velocities, during sizing to verify that a duct diameter keeps velocity within acceptable limits, and during troubleshooting to confirm what velocity a given duct is seeing.
Use when You have any two of CFM, diameter, or velocity and need the third.
Variables
CFM Airflow in cubic feet per minuteDiameter Round duct inside diameter in inchesVelocity Air velocity in feet per minute (FPM)
Solve for
Diameter (in)
Velocity (FPM)
Air velocity in feet per minute
CFM —
Friction Loss (in.w.g./100ft) —

What is duct air velocity?

Duct air velocity is the speed at which air moves through a duct, measured in feet per minute (FPM). It is calculated as: Velocity = CFM ÷ Duct Area (ft²). Velocity directly affects noise levels, pressure drop, and energy consumption in a duct system. Higher velocities increase friction losses (which follow a square-law relationship with velocity) and generate more aerodynamic noise. ASHRAE applications guidance recommends keeping main duct velocities below 2,000 FPM in occupied areas to control noise, with branch ducts at 600–1,200 FPM depending on the application. Industrial ductwork for exhaust systems may operate at higher velocities to maintain transport velocity for particulate. Duct sizing balances first cost (smaller ducts are cheaper) against operating cost (higher velocity means more fan energy). The equal-friction method and static-regain method are common duct sizing approaches that target specific velocity and pressure drop criteria.

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