CO2 Consumption in a Brewery
A barrel is 4.2 cubic feet. So at sea level and 70 deg. F a 5 barrel tank will have 21 cubic feet of gas in it when the gauge reads 0 psi. The volume of gas required to take that tank up to 14.7 (1 atmosphere) will be another 21 cubic feet.
In other words, the volume of gas used (assuming no waste) is 4.2 cubic feet per atmosphere of pressure. The math looks like this: number of barrels x 4.2 x atmospheres = total volume of system. Remember that when the gauge reads 0 at sea level the pressure is already at 1 atmosphere.
Source: Dan @ McDantim