The fill-time formula, with units
Free air (ft³) = tank volume (ft³) × (end PSI − start PSI) ÷ atmospheric PSIAFill seconds = free air ÷ (pump CFM − air-use CFM) × 60Retained CFM = free air × 60 ÷ measured secondsOne US gallon equals 231 cubic inches, so the calculator uses 231 ÷ 1,728 cubic feet per gallon. One cubic foot is 28.316846592 liters. The pressure difference uses two gauge readings; atmospheric pressure is an absolute value. Do not add 14.7 to the pressure difference.
The storage relation follows the constant-temperature receiver model described by CAGI. Reversing it for a rising pressure and dividing by net flow gives the ideal filling-time equation. This is our calculation from that model, not a manufacturer’s tested recovery-time claim.
Worked example: a 20-gallon tank from 90 to 120 PSI
At 14.7 PSIA atmosphere, that 30 PSI rise stores about 5.456 cubic feet of equivalent free air. With an assumed constant 5 CFM coming in and no air being used, the estimated pump-on time is 65.5 seconds.
If another application uses 2 CFM during the same interval, the net filling flow becomes 3 CFM and the estimate rises to 109.1 seconds. If concurrent use reaches 5 CFM, the model has no positive refill flow and no finite fill time. Increasing tank size does not resolve that supply deficit.
Calculate CFM from tank fill time
Suppose a known 60-US-gallon receiver rises from 90 to 120 PSI in 90 seconds, with no intended air use. The same model gives about 10.91 CFM net retained over that interval. It does not establish “10.91 CFM at 90 PSI,” because the pressure changes throughout the measurement.
Record the same start and end pressures when comparing observations. Include any connected receiver volume you can establish. Follow normal compressor operation and the manufacturer’s instructions; changing the pressure switch or exceeding a rating is not part of this method.
Why the real stopwatch result can differ
- Delivery varies with pressure. A single 90 PSI specification is not necessarily the average delivery from 0 to maximum pressure.
- Temperature changes pressure. Air warming during compression can make a simple equal-temperature estimate read high; cooling and leaks can also change the result.
- Air may leave the tank. A leak or operating tool means retained airflow is lower than incoming pump flow under the same model.
- Motor rests add elapsed time. The result is ideal pump-on time, not permission to exceed the manual’s duty or continuous-run limit.
Use repeated observations as a comparison aid, not an ISO-rated SCFM test or a fault diagnosis. See the tank buffer guide for the corresponding pressure-drop question and duty-cycle guide for run/rest definitions.
Common questions
Can this estimate pump-up time from zero PSI?
Yes. Enter zero as starting gauge pressure and the permitted target as ending pressure. The flow input must represent the entire interval; a single published delivery point may be a poor approximation.
Is the timed CFM result the compressor’s rated SCFM?
No. It is net retained airflow under the stated atmospheric and temperature assumptions. A certified rating requires specified test conditions and procedures.
Why does doubling the tank double the estimated fill time?
With the same pressure rise and constant net flow, twice the receiver volume requires twice the equivalent free air. Real equipment can differ because flow, temperature and controls change.