Propane Tank Size Converter

Propane is sold by weight, used by volume, and stored in tanks measured both ways. That mismatch causes endless confusion: a “20 lb” BBQ tank holds about 4.7 gallons, weighs roughly 38 lbs when full, and contains around 432,000 BTU of energy. Different number for the same physical object, depending on who’s asking.

The basic conversions (propane at 60°F):

Every figure in that table comes from just two measurements: 4.24 pounds per gallon and 91,502 BTU per gallon, both for liquid propane at 60°F. Everything else is division. Working that way is the only reason the pounds route and the gallons route give the same answer, and it is worth checking on any propane chart you find: multiply the BTU-per-pound figure by the pounds-per-gallon figure and see whether you land back on the BTU-per-gallon one.

You will also see 21,548 BTU per pound quoted, which comes from a slightly different density measurement. The gap is 0.15%, about 30 BTU on a pound, and it makes no difference to sizing a tank.

These numbers all reference liquid propane at room temperature. Propane is stored as a liquid and used as a gas, and the vaporization happens inside the tank as gas is drawn off the top.

Standard tank sizes:

The 80% fill rule. Propane tanks are filled to 80% of water capacity to leave room for thermal expansion of the liquid. As temperature rises from 70°F to 120°F, propane volume increases by roughly 7 to 8%. Tanks filled to capacity (above 80%) can vent at the pressure-relief valve in summer heat, a frequent source of garage and RV propane fires.

Reading a tank label.

US propane tanks are stamped with three key markings:

• WC (Water Capacity): the total volume in pounds of water
• TW (Tare Weight): the empty weight of the tank
• DOT/TC number: the manufacturer/inspector code

The propane capacity works out to roughly 40% of WC in pounds: propane is about half as dense as water (0.504), and the fill stops at 80% of the volume. US filling regulations set the exact weight-fill limit at 42% of WC, which is why a tank stamped WC 47.6 takes 20 lb of propane.

Tank recertification.

US propane tanks require recertification 12 years from manufacture, then every 5 years after. Look for the date stamp on the collar; uncertified tanks cannot be filled by most refill stations.

Refill economics.

Refilling at a propane filler station (Tractor Supply, U-Haul, hardware stores) costs about half what a tank swap costs. A 20 lb tank refill runs $15-25 USD; a tank swap (Blue Rhino, AmeriGas exchange) runs $25-40 and usually shorts you about 5 lb of propane: the exchange tanks are filled to 15 lb of propane, not the 20 lb a full refill provides.

Approximate runtime by appliance:

These are full-output hours, and real-world use is usually intermittent, so practical runtime is longer. A 20 lb tank holds about 432,000 BTU, so any row here is that figure divided by the appliance’s burn rate.

Choosing a tank size. For occasional grilling, a 20 lb tank is fine. For generator backup, get a 100 lb tank or larger, because refilling a generator during a power outage is a miserable job. For whole-home heating, you need a permanent 250 to 1,000 gallon tank installed by a licensed propane service.

Cold weather and propane.

At low temperatures, propane vaporization slows. A tank in 0°F weather may not deliver enough gas pressure to run a high-BTU appliance. Larger tanks have more surface area and vaporize faster; small bottles can frost over and starve appliances of fuel. Below 20°F, plan to use larger tanks or to keep small tanks insulated.