Duct Velocity Calculator

Work out duct velocity instantly with clear inputs, formula shown and shareable results.

About Duct Velocity Calculator

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.

Formulas

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

Worked example

With the default values below, Duct Velocity Calculator returns Air velocity of 1,029. Change any input above and every figure here is recalculated in your browser.

Inputs used

Airflowminimum 1 CFM: 800 CFM
Duct widthminimum 1 in: 14 in
Duct heightminimum 1 in: 8 in

Results

Air velocity: 1,029 fpm
Cross-sectional area: 0.778 ft²
Equivalent round diameter: 11.5 in
Velocity pressure: 0.0660 in.wg

Step by step

1Values used
2Velocity
3Equivalent round
4Velocity pressure
5Air velocity= 1,029 fpm
6Cross-sectional area= 0.778 ft²
7Equivalent round diameter= 11.5 in
8Velocity pressure= 0.0660 in.wg

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Duct Velocity figure worked out?

V fpm = CFM / area ft², area = width x height / 144. Velocity is airflow divided by free area.

Why does velocity matter beyond fitting the duct in?

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.

What values do I need to enter?

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.