Propane is a widely used fuel source, but its measurement can be confusing because its state changes depending on temperature and pressure. It is stored and transported as a pressurized liquid before turning into a gas for use in appliances like grills and furnaces. This transformation complicates determining the weight of a gallon. The weight calculation focuses on the liquid’s density, as the weight of the gas is negligible.
The Specific Weight of Liquid Propane
A standard gallon of liquid propane has a specific weight when measured at a baseline temperature. At 60 degrees Fahrenheit, one gallon of liquid propane weighs approximately 4.2 pounds. This measurement applies while the fuel is compressed in its liquid state inside a tank. The amount of usable energy in propane is directly related to this mass.
Propane is considerably lighter than other common liquids. For comparison, a US gallon of water weighs about 8.34 pounds at room temperature. This means liquid propane is less than half the density of water. The standard industry figure for calculations is 4.2 pounds per gallon.
The Influence of Temperature on Propane Density
The weight of a gallon of propane is not entirely fixed because density changes with temperature. Density dictates how much a specific volume of any substance weighs. As propane is heated, its molecules move faster and spread further apart, causing the liquid to expand.
This expansion means a gallon of warm propane occupies more space and weighs less than a gallon of cold propane. Conversely, colder temperatures cause the liquid to contract, resulting in higher density and greater weight per gallon. The industry standard of 4.2 pounds per gallon is tied to the baseline temperature of 60 degrees Fahrenheit.
Translating Gallons into Standard Tank Capacities
The weight-per-gallon figure determines how much fuel can be stored in common consumer tanks. For instance, a typical barbecue grill tank is rated to hold 20 pounds of propane. Based on the standard 4.2 pounds per gallon, this 20-pound capacity translates to approximately 4.7 gallons of liquid propane.
An important safety measure requires that propane tanks are never filled to their total physical capacity. The mandated 80% fill rule ensures 20% of the tank volume remains empty to accommodate thermal expansion. This empty space prevents dangerous pressure buildup if the liquid propane expands due to high ambient temperatures. The tank’s total capacity is higher than the fill weight to meet this safety margin.
Why Propane is Sold by Weight, Not Volume
Propane is sold by weight, in pounds, rather than by volume, in gallons, for reasons of consistency and consumer protection. This practice addresses the significant fluctuations in the liquid’s volume caused by temperature changes. Selling by weight ensures that the customer receives the exact mass of fuel, which represents consistent energy content, regardless of the ambient temperature on the day of delivery.
If propane were sold strictly by the gallon, a customer would receive less actual fuel mass on a hot day when the liquid is expanded and less dense. By using a scale to measure the mass added to a tank, suppliers provide an accurate, standardized amount. For smaller cylinders, the weight measurement is straightforward and relates directly to the tank’s stamped weight rating.