A 500-gallon propane tank is a steel storage tank that holds liquid propane gas for residential use. The 500-gallon term means the tank has a water capacity of 500 gallons, according to Midway Gas. It must be safely located outside and away from the house.
Like water, propane expands and contracts with changes in the temperature; however, propane can expand up to 17 times the amount that water does at the same temperature. For safety a propane tank is filled only to 80 to 85 percent of its total capacity, or 425 gallons; this allows space for expansion and contraction. A propane tank has a gauge to show approximately how much propane is left in the tank and a hand wheel to cut off the supply of propane to the house.