Two capacities to compare
A tank stores pressurized air. A pump replenishes air. Increasing one does not automatically increase the other. That distinction matters when two compressors share an airflow rating but have different receivers: the larger tank can provide a larger theoretical buffer over the same pressure interval.
This calculator intentionally examines the pump-off case. It estimates how long a specified, continuous free-air demand would draw the tank from the chosen start pressure to the chosen end pressure. It does not calculate how long an idle tank retains pressure.
A transparent, hypothetical example
Using a constant-temperature approximation at a 14.7 PSI atmosphere, usable free-air volume equals tank volume in cubic feet multiplied by the pressure difference, divided by 14.7. Divide that volume by the demand in cubic feet per minute, then multiply by 60 for seconds. One cubic foot is approximately 7.48052 US gallons.
For a hypothetical 10-gallon tank, a 120-to-90 PSI interval and 20 CFM continuous demand, the result is approximately 8.2 seconds with the pump off. At the same conditions, 20 gallons gives approximately 16.4 seconds. Doubling the tank doubles this theoretical quantity; doubling demand halves the time.
What the example does not predict
A working compressor may start during the drawdown, while tool demand and delivered flow can change. Temperature, altitude, regulation, hose restrictions and leakage can also affect the result. The displayed seconds are therefore not measured working time, recovery time or a guarantee that the tool maintains its required pressure.
Use identical assumptions when comparing tank sizes. Review pump delivery and duty limits first, then ask whether the larger buffer is worth the extra space. Do not raise the chosen start pressure above the receiver’s permitted rating or interpret the hypothetical interval as a pressure-switch adjustment.
Tank volume changes the stored-air buffer. Published pump delivery determines how quickly air can be supplied under its stated conditions.