Pump-Up Test CFM Calculator

How to run the test so the number means something

1. Close the service valve so nothing downstream is drawing air. If the tank has a leak you can hear, fix that first or the test measures the leak too.
2. Drain the tank down to your start pressure. Pick a window the pump spends real time in, 90 to 120 psi on a 125 psi machine, 100 to 150 on a 175 psi two stage. Avoid starting from zero, the first few psi fill fast and flatter the result.
3. Start the stopwatch the moment the needle passes the start pressure and stop it when it passes the end pressure. Two readings, one time. Do it twice and use the average.
4. Read the tank size off the data plate, not off the catalog. Some 80 gallon tanks are 80, some are 72.

What the number is, and isn't

It's free air delivered, in cubic feet per minute at atmospheric pressure, worked out from how much the tank's contents grew. That is the same basis manufacturers rate pumps on, so it's a fair comparison to the catalog figure, usually within five percent once the tank and the air in it have warmed up. It is not SCFM corrected to a standard temperature, and it doesn't account for the air in the pipe between pump and tank. For deciding whether a pump is worn, neither matters.

Where the healthy figure comes from

Not a textbook. It's the median rated output of every two stage piston compressor we sell at that horsepower, 174 variants at 5 HP, 278 at 7.5, 152 at 10. The median 5 HP two stage on our shelves makes 17 CFM; the spread runs from 11.4 to 34.8 because a 5 HP motor can turn a small pump fast or a big pump slow. So treat the percentage as a strong hint, and if your pump's own rating is printed on it, compare to that instead. Single stage pumps run about 2.65 CFM per horsepower across our catalog against 3.25 for two stage, which is the whole argument for two stage in one line.

Reading the percentage

Above about 90 percent, the pump is fine and any air shortage is downstream: leaks, undersized pipe, a tool that wants more than you have. Between 70 and 90, it's doing work but tired, and on a piston pump that's usually valves. Reed or finger valves are cheap and an afternoon's job. Below 70, think rings and cylinder wear, and at that point a new bare pump on your existing tank and motor is often cheaper than the rebuild. If the number is wildly low, below half, suspect the test before the pump: a service valve left open, a leaking tank drain, or an unloader that didn't seat.

Running this the other way, with a known CFM to estimate fill time, uses the same math and the same caveats. Full range of machines is under shop by CFM.