Air Compressor Calculator
Check whether your air compressor's tank and CFM match your tools, and how long the tank takes to refill.
Inputs
Formula
- Usable air = tank volume × (P_max − P_min) ÷ atmospheric pressure
- Run time on tank alone = usable air ÷ tool consumption
- Refill time = usable air ÷ compressor output
A compressor tank stores compressed air. The air you can actually use is the tank volume multiplied by the pressure drop between cut-out and cut-in, expressed in atmospheres. If the tool uses more CFM than the compressor makes, the tank only buys you a few seconds and the motor will run continuously.
Step-by-step calculation
- 1
Pressure drop
P_max − P_min
2 bar
- 2
Stored air
tank × Δp ÷ 1.013
98.7 L free air
- 3
Run time
stored ÷ tool CFM
52 s
Worked example
- 1A 50 litre tank between 8 and 6 bar stores 50 × 2 ÷ 1.013 = 98.7 free litres = 3.49 cu ft.
- 2A 4 CFM spray gun empties that in about 52 seconds.
- 3A 6 CFM compressor refills it in about 35 seconds.
Tips
- Choose a compressor with at least 1.5 times your tool's CFM for continuous work.
- Drain the tank daily — condensed water rusts it from the inside.
Frequently asked questions
What CFM do I need?
Check the tool plate: a nail gun needs 2 CFM, an impact wrench 4–5 CFM, a spray gun 6–10 CFM.
Does a bigger tank replace CFM?
No. A big tank helps with short bursts, but sustained work needs enough CFM output.
What pressure should I set?
Most air tools need 6.2 bar (90 psi) at the tool, so set the regulator slightly higher to cover hose loss.
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