= V (tank size) x (final pressure – initial pressure)
7.48 x atmospheric pressure x pump delivery (CFM)

Example: 7.5 HP compressor rated at 24 CFM with an 80 gallon tank, unit starts at 100 PSI and turns off at 150 PSI

80 gallons x (150 PSI – 100 PSI) = 4000 = 1.51 minutes
7.48 x 14.7 x 24 CFM 2638