The approximate amount of gas (cubic feet of gas) in standard conditions in 1000 feet of pipeline can be calculated using the following formula.

Amount of Gas (ft3) = d2 * P * 0.372

Where:
d = inside diameter of piping
P = pressure (psi)

Example: How much gas in 1000 feet of 16" schedule 30 pipe at 350 pounds pressure?

16" schedule 30 pipe wall thickness is 0.375"; so inside diameter of 16" pipe is 15.25".

Amount of Gas = (15.25)2 * 350 * 0.372 = 30,280 cubic feet of gas

Another Example: Approximately how much gas in eight miles of 0.25" wall 24-inch pipe if the pressure is 400 psi?

Amount of Gas = (23.5)2 * 400 * (8 miles * 5,280 ft/mile) * 0.372 = 3,470,000 cubic feet of gas

When a section of pipeline is blown down from one pressure to another, the total gas lost may be computed by the difference in the contents at the two pressures, using the above rule.