How Many Gallons Are in a 20 lb Propane Tank?

Small propane tanks are named for the weight of propane they hold, not for their volume. Ferrellgas notes that a standard grill tank holds 20 pounds of propane when full.

A gallon of liquid propane weighs 4.24 lb, according to AmeriGas, so 20 pounds of propane comes to about 4.7 gallons.

The tank itself adds weight. In the PERC example, the empty cylinder weighs 18 lb, so a full one weighs 38 lb.

Both figures come from the propane industry. The Propane Education & Research Council (PERC) gives 4.7 gal as an approximate capacity, while AmeriGas says 4.6 gal and Suburban Propane says about 4.6 gal.

The weight math lands closer to the higher figure. Dividing 20 lb by 4.2, the factor Ferrellgas uses, gives 4.76 gallons. The gap between the two published figures is only about a tenth of a gallon.

Propane tanks are never filled to the top. AmeriGas says a tank can only be filled to 80%, and Suburban Propane explains that filling to about 80% leaves room for the propane to expand and contract safely as temperatures change.

Small cylinders guard that space too. The Propane Education & Research Council says an overfill prevention device keeps a small cylinder from being overfilled, so the liquid has room to expand in warm weather.

Larger tanks are named in gallons, but the name gives the size of the whole tank, not the propane inside. AmeriGas lists a 1,000 gal tank as holding 800 gal when full.

A 120 gal tank is also called a 420 lb tank, and it holds 96 gal of propane. To turn any of these figures into liters, use the gallons to liters converter.

Weigh the tank. Ferrellgas suggests putting it on a scale, subtracting the empty weight stamped on the tank, and dividing the pounds of propane left by 4.2 to get gallons.

For example, 10 lb of propane is about 2.4 gallons, or half of a full twenty-pound tank.