Air Compressor Fill Time Calculator
Enter your compressor pump CFM, tank volume, and pressure switch settings to get the fill time and whether it can keep up with your tool's continuous air demand. Nothing uploaded.
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Learn more: fill time, CFM, and why tools stop keeping up
How the fill time is worked out
Filling a tank from cut-in to cut-out pressure means pumping in a volume of free air, meaning air measured at atmospheric pressure. The calculation multiplies tank volume by the pressure rise in bar and divides by atmospheric pressure, held at 1.0135 bar.
With the default 50 litre tank cycling between 90 and 120 PSI, that pressure rise is about 2.07 bar, so the pump has to move 102 litres of free air. A 6 CFM pump delivers 170 litres a minute, which puts the fill at 0.6 minutes, or 36 seconds.
Both sides of that scale linearly. Double the tank to 100 litres and the free air needed doubles to 204 litres, taking the fill to 72 seconds with no change in how fast the pump works.
CFM and SCFM are not the same label
The duty cycle check compares your pump's CFM against your tool's CFM directly, with no correction applied between them. Both numbers have to be quoted on the same basis for the answer to mean anything.
Rolair Systems' explanation of PSI, CFM and air fittings describes SCFM as the more exact measurement, one that "attempts to arrive at a more accurate measurement by ruling out differences in air temperature and humidity". Plain CFM ratings are instead quoted at a stated pressure, written out like 8 CFM at 90 PSI, and two ratings taken at different pressures do not compare directly.
Duty cycle and the 20% headroom rule
Duty cycle here is tool demand divided by pump output, capped at 100%. The default 4 CFM tool on a 6 CFM pump comes out at 67%.
The verdict asks for more than parity. A pump has to deliver at least 1.2 times the tool's demand to read as keeping up, so a 6 CFM pump clears a 5 CFM tool at 83% duty cycle. Past that point the verdict turns marginal with a note to watch temperature, and below parity it says the compressor cannot keep up at all.
FAQ
Why does a bigger tank help if the pump is the same?
A bigger tank holds more air at pressure, so short bursts from something like a nail gun draw on stored air instead of waiting on the pump. The pump still has to catch up afterwards, and the calculator shows what that costs: doubling tank volume doubles the fill time for the same pump.
What does the duty cycle percentage mean?
It is the share of the time the pump has to run to keep up with the tool you entered, worked out as tool CFM divided by pump CFM. Values near 100% mean the pump runs almost continuously with little chance to cool, which shortens its life.
Does altitude change these numbers?
The calculation assumes sea-level atmospheric pressure at a fixed 1.0135 bar. Real atmospheric pressure falls with altitude, which raises the volume of free air a fill needs, so a real fill at high elevation takes a little longer than the number shown.