The SI unit for volume is. The formula for the volume of a cylinder. The volume of a cylinder is 314 pi times half the diameter squared times itself multiplied by the height of the cylinder.
You now have the volume of the cylinder in the same units you used to measure the parts.
P1 is the pressure of the compressed gas in the cylinder psi V1 is the internal volume of the cylinder often referred to as water volume liter P2 is the atmospheric pressure 1 atm - 147 psi V2 is the volume of gas at pressure P 2 liter For example an AL sized cylinder is filled with nitrogen at 2000 psi. Multiply the area of the cylinder base the circle by the height of the cylinder. To be able to picture more easily how much water it actually is lets convert that value to liters. 2 x 4 in.