Air Compressor CFM Test Calculator: Check Real Output

The CFM printed on the box is a rating, not a promise. This calculator works out what your compressor actually delivers from a simple timed pump-up test: note the tank size, the pressure the needle passes when you start the stopwatch, the pressure when you stop it, and the seconds in between. Compare the answer with the rating and you can tell a healthy pump from one with worn rings, a leaking check valve or a hidden air leak.

How to run the test

1. Close the outlet valve or unplug every hose and tool, so the only thing taking air is the tank.
2. Open the drain valve and let the tank bleed down below your start pressure, then close the drain.
3. Switch on. When the gauge needle passes your start pressure (90 PSI is a good choice, because that is where most CFM ratings are quoted), start a stopwatch.
4. Stop the watch at your end pressure, or the moment the pump cuts out, and note that pressure.
5. Enter the numbers above. Run it two or three times and use the average, because reading a moving needle is never exact.

Use the tank gauge, not the regulated outlet gauge. Starting the timer above zero also skips the first few seconds while the motor gets up to speed, which would otherwise make a healthy pump look slow.

How the math works

Raising the pressure in a fixed tank takes a known amount of free air: tank volume in cubic feet, times the pressure rise in PSI, divided by 14.7 (atmospheric pressure). One US gallon is 0.1337 cubic feet. Divide that air by the minutes it took and you have the cubic feet per minute your pump really pushed into the tank. With the default figures, a 20 gallon tank rising 30 PSI holds about 5.46 cubic feet of extra free air; if that took 75 seconds, the pump is delivering about 4.4 CFM.

Reading the result

Expect the measured figure to sit a little under the rating. Output drops as tank pressure climbs, a hot pump moves less air than a cold one, and labeled tank sizes are often rounded. Our rough guide: within about 10% of the rating is a healthy machine; 10 to 25% short is worth investigating; more than 25% short usually points to a fault. The common causes are worn piston rings or reed valves, a leaking check valve, a dirty intake filter, or air leaking from a fitting, the drain or the pressure switch. If the numbers look fine but tools still starve, check the hose with our hose pressure drop calculator, and confirm what your tools need with the air compressor size calculator.

Want the opposite answer, how long a pump of a given CFM should take to fill a tank? Use the fill time calculator.