Propane gas is used in portable appliances and residential applications in areas where connection to a natural gas line is not available. Usually sold by the pound when dispensed into portable tanks, propane gas is sold by the gallon when weighing the tank isn't feasible. When converting propane gas between pounds and gallons, use either the weight or volume of gas along with the standard weight of a gallon of propane.
Weigh a portable tank before filling to account for the weight of the tank as well as any gas that may be left in the tank.
Fill a portable tank with gas and reweigh the tank. Subtract the beginning weight of the tank from the tank's filled weight. As an example, if the beginning weight of the tank was 25 pounds and the filled weight is 45 pounds, then 20 pounds of gas were added to the tank (45 – 25 = 20).
Convert pounds to gallons. As propane gas weighs 4.2 pounds per gallon, you will need to divide the total amount of gas dispensed in pounds by 4.2. This results in 20 punds of propane equaling approximately 4 3/4 gallons of gas (20 ÷ 4.2 = 4.76).
Reverse the calculations to determine the amount of gas in pounds if dispensing propane in gallons. For example, 50 gallons of propane gas would convert to 210 pounds of propane (50 x 4.2 = 210).