Duct Velocity Calculator (Round & Rectangular)

Velocity = airflow ÷ free area. Too high causes noise; too low wastes duct material.

HVAC Pro calculator — inputs, formula and example are open to everyone.

Result

Results for this calculator are part of HVAC Pro

You can see the inputs, formula and worked example. Unlock results for every Pro calculator.

Unlock with HVAC Pro

Already a member? Sign in

Formula

V (fpm) = CFM ÷ A (ft²)

Example calculation

400 CFM in an 8 in round duct (0.349 ft²) → 1,146 fpm.

Practical HVAC use

Check noise and branch sizing.

Common mistakes

Using outside dimensions of lined duct.

Related calculators

Round Duct Sizing

Pro

Size a round duct for a target velocity from airflow, with metric output and nearest standard size.

Duct Friction Loss

Pro

Estimate friction loss per 100 ft and for a duct run from airflow and round duct diameter.

CFM ↔ L/s ↔ m³/h

Free

Convert CFM to litres per second and cubic metres per hour. Free airflow unit converter for HVAC ventilation and duct design.

Educational tool. Results are estimates from the stated formulas; always confirm with manufacturer data and the code adopted in your jurisdiction. All calculators