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: compressor fill time, CFM, and duty cycle
How to calculate air compressor fill time
Fill time depends on the volume of free air needed and the pump's output rate. Free air volume = tank volume × (cut-out pressure - cut-in pressure) / atmospheric pressure. Divide by pump LPM to get fill time in minutes. A 50L tank filling from 6 to 8 bar with a 170 LPM pump takes approximately 3-4 minutes. This calculator automates the formula.
What CFM and FAD mean for compressors
CFM (cubic feet per minute) is the volume of free air (at atmospheric pressure) that the compressor pump delivers per minute. This is the FAD (Free Air Delivery) rating on the spec sheet. It is distinct from the displacement of the pump - actual FAD is lower due to compression efficiency losses. Higher CFM means you can run more demanding tools or run them continuously.
Will my compressor keep up with my tools?
Compare the tool's CFM requirement (found on the tool spec sheet or handle) to the compressor's FAD CFM. For continuous use tools like sanders or grinders, the compressor FAD should exceed the tool requirement by at least 20%. An undersized compressor either can't fill fast enough or runs so continuously that it overheats. The calculator shows the duty cycle percentage - above 80% risks overheating the motor.
Tank pressure and cut-in/cut-out settings
Most compressors operate between a cut-in pressure (e.g. 6 bar) when the motor kicks on, and a cut-out pressure (e.g. 8 bar) when the motor shuts off. The larger this differential, the longer the fill time but the less often the motor cycles. Many portable compressors use 6-8 bar; larger industrial units may use 8-10 bar.
FAQ
How do I calculate air compressor fill time?
Free air needed = tank volume × (cut-out - cut-in) bar / 1.0135 bar. Divide by pump L/min for fill time in minutes. Enter your values and the calculator does this automatically.
What does CFM mean for compressors?
CFM = cubic feet per minute of free air delivered at atmospheric pressure. The FAD (Free Air Delivery) rating on the spec sheet. Higher CFM means more air per minute and ability to run more demanding tools.
How do I know if my compressor can keep up?
Compare your compressor's FAD CFM to the tool's demand. For continuous tools, FAD should exceed demand by 20%+. The calculator shows duty cycle - above 80% risks overheating the motor.