CFM Calculator

CFM Calculator works out the airflow inside a duct — how many cubic feet per minute a given duct size carries at a given air velocity, what that costs in friction and static pressure, and whether the blower on the other end can actually push it. Enter the duct dimensions and the velocity and read the CFM straight away. It works completely offline and nothing you enter ever leaves your phone.

HOW IT WORKS
The app uses the standard duct-design method:

1. Airflow is velocity times area: Q = V x A. A round duct's cross-section is pi/4 x (diameter in inches / 12) squared, in square feet; a rectangular one is width x height / 144. Feet per minute times square feet gives cubic feet per minute.

2. A rectangular duct is looked up as its circular equivalent, the round duct that loses the same pressure at the same airflow: De = 1.30 x (a x b)^0.625 / (a + b)^0.25, from the ASHRAE Handbook. It is not the diameter of equal area, because a flat rectangle has far more wall touching the same air.

3. Friction loss per 100 feet comes from Darcy-Weisbach with a Colebrook-White friction factor, on standard air at 0.075 lb per cubic foot and a galvanised duct wall. Velocity pressure is the familiar (velocity / 4005) squared, in inches of water gauge.

4. The loss over the whole run is priced against the TOTAL EFFECTIVE LENGTH — the straight duct plus the equivalent length of every elbow, boot and take-off. The friction rate the duct may spend is the available static pressure x 100 / total effective length.

5. Cooling airflow is tons x CFM per ton, where one ton is 12,000 BTU/h and about 400 CFM per ton is nominal.

A 14 inch round duct is 1.069 square feet, so 900 feet per minute through it is 962 CFM — 1,635 cubic metres an hour, or 454 litres a second — losing about 0.082 inches of water per 100 feet.

WHAT YOU CAN DO
- Enter a round diameter or a rectangular width and height and read the CFM instantly
- See the circular equivalent diameter of any rectangular duct
- Slide the air velocity from 200 to 2,500 fpm and watch airflow, velocity pressure and friction move with it
- Set the straight length, the fitting equivalent length and the static pressure available, and see the loss over the whole run
- Find the CFM the duct will actually carry at its design friction rate, and how far short a too-small duct falls
- Work out the blower airflow for a cooling load at 300 to 500 CFM per ton, and the CFM per supply register
- Try the fan laws: airflow with RPM, static pressure with RPM squared, power with RPM cubed
- Convert between CFM, cubic metres per hour, litres per second, feet per minute and metres per second
- Open Details for a full step-by-step breakdown and a table of recommended duct velocities
- Learn the maths and the practice behind the numbers in plain language

BUILT FOR CLARITY
A cool plant-room green design puts the big airflow figure first, with velocity, friction and pressure right beside it. Light and dark themes, a tiny download, no clutter and no account required.

AN ESTIMATE, NOT A MECHANICAL DESIGN
This is a simplified educational tool. It assumes standard air, smooth galvanised duct and steady flow. Flexible duct is several times rougher, leakage and altitude and air temperature all shift the answer, and a real system is sized to ACCA Manual D or the ASHRAE and SMACNA duct-design chapters, against the manufacturer's own fan curve. Airflow should always be confirmed by measurement, and the design confirmed by a qualified HVAC professional working to your local mechanical code. This app is for general information and education only and is not professional engineering advice.

PRIVACY
The app collects no personal data and works fully offline. Ads are served by Google AdMob, which may use an advertising identifier as described in Google's policies.

Download CFM Calculator and get a quick, honest picture of the air your ductwork can really move.