Work out duct velocity instantly with clear inputs, formula shown and shareable results.
Velocity is airflow divided by free area. For a rectangular duct the equivalent round diameter from the Huebscher relation lets you read friction charts drawn for round duct, and velocity pressure, (V/4005)², is the dynamic head used when converting fitting loss coefficients into inches of water gauge.
Velocity
V fpm = CFM / area ft², area = width x height / 144
Equivalent round
De = 1.30 x (a x b)^0.625 / (a + b)^0.25
Velocity pressure
Vp = (V / 4005)² in.wg
V fpm = CFM / area ft², area = width x height / 144. Velocity is airflow divided by free area.
Friction loss grows roughly with the square of velocity and noise grows faster still, so a duct that is one size too small costs fan power permanently and is often audible.
This calculator takes 3 inputs: Airflow, Duct width, Duct height. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.