Compressed-air arithmetic runs in two different volumes, and free air is the one demand is measured in: cubic feet (or m³) at atmospheric conditions, as the tools actually consume it, rather than the compressed volume sitting in the tank. The conversion is what atmospheric pressure is doing inside the standard receiver formula — each tank volume releases (P1 − P2)/Patm tank-volumes of free air as it blows down through its pressure band, so the usable stored free air is Va = V × (P1 − P2)/Patm. A 240-gallon receiver (32.1 ft³) drawn from 125 down to 100 psig at sea level holds 32.1 × 25/14.7 = 54.6 cubic feet of atmospheric-equivalent air.
Working in free air is also what makes altitude matter. At 5,000 ft, where atmospheric pressure is about 12.2 psia instead of 14.7, the same pressure band stores roughly 20% more free-air equivalents per gallon of tank — which is why the receiver-sizing formula t = V(P1 − P2)/(Patm(Qd − Qs)) carries an editable atmospheric pressure, and why both the demand of an event and any concurrent compressor supply enter it as free-air flow in cfm or m³/min.