There are 5 gallons in a 20 pound propane tank. This is why 20 pound tanks are sometimes also referred to as 5 gallon tanks.
A 20 pound tank of propane is typically used for gas grills. There are about 21,591 btus (British Thermal Units) of fuel in every pound of gas. So a 20 pound tank has about 431,820 btus of fuel. Once the maximum btu output of the grill burner is determined, the total amount of fuel in the tank can be divided by the burner output to determine the number of hours a 20 pound tank of propane will last.