How Long Does a 20 lb Propane Tank Last? Appliance Math (2026)

A full 20-pound propane tank lasts about 18 to 20 hours on a medium gas grill, about 14 hours on a 30,000 BTU patio heater, and about 48 hours on a portable buddy heater on high. Every one of those numbers comes from the same division: roughly 430,000 BTU in a full tank divided by the appliance's BTU rating.

The short version:

• A full 20-pound tank holds about 430,000 BTU, or about 4.7 gallons at 4.24 pounds per gallon.
• Hours = 430,000 divided by the appliance's BTU rating, printed on its label or manual.
• Grill on medium (about 25,000 BTU): 17 to 20 hours. All burners on high (about 40,000 BTU): 10 to 11 hours.
• Exchange tanks are often filled to 15 pounds, not 20, so knock about a quarter off every number below unless you refilled at a dealer.

How long does a 20 lb propane tank last?

It depends on the burner, not the tank. Applied to the appliances people actually own:

Appliance	Typical BTU rating	Hours on a full 20 lb tank	What it covers
Gas grill, medium	25,000	17 to 20	A season of weeknight grilling
Gas grill, all burners high	40,000	10 to 11	One long barbecue weekend
Patio heater	30,000 to 50,000	9 to 14	Two or three cold evenings
Portable buddy heater, high	9,000	About 48	Six nights of bedroom heat
Portable buddy heater, low	4,000	About 100	Two weeks of overnight warmth
Portable generator, half load	Varies	7 to 10	An overnight essentials run

The generator row is its own topic, with tank-size scaling up to whole-home cylinders, and the propane generator runtime guide works through that math. Everything else on the table follows the same formula.

The BTU math behind every estimate

Propane carries about 21,500 BTU per pound, so 20 pounds works out to roughly 430,000 BTU. Find the appliance rating on its label, divide, and you have the runtime: 430,000 divided by 30,000 is a bit over 14 hours for that patio heater, and 430,000 divided by 9,000 is about 48 hours for a buddy heater on high. Published grill figures land in the same place: manufacturers and grill guides consistently quote 18 to 20 hours of medium-heat grilling per tank, which matches the division.

Two things move the number in real life. First, the fill: a dealer refill pumps the full 20 pounds, while exchange cages typically fill to 15, so an exchange tank is really about 320,000 BTU. Second, the cold: propane is stored as a liquid and burned as a gas, and cold slows the boil between the two, so a small tank feeding a big load on a freezing night can sputter before it is truly empty. Keep the cylinder upright on a dry base out of the wind, and size up for deep-winter duty.

Making one tank cover an outage

For cooking, a grill sips: an hour of medium-heat dinner cooking barely dents the tank, which is why one full cylinder covers weeks of normal grilling. For heat, the setting is everything: a buddy heater on low stretches the same tank past 100 hours, more than twice its high-setting runtime, and heating one room instead of the house multiplies that advantage. Pair the heater with a 20-pound tank through a hose assembly instead of burning through one-pound canisters, and a single refillable cylinder becomes a serious outage asset. The winter heating safety guide covers ventilation, clearances, and CO alarms for running one indoors.

Handle the tank itself with the boring rules that prevent the exciting failures. Swap tanks with burners off and cooled, hand-tighten the fitting, and leak-check with soapy water, never a flame. Spare cylinders live outdoors or in a ventilated outbuilding, upright, away from exhaust and ignition sources. Unlike gasoline, which degrades in 30 to 90 days untreated and needs the full storage routine, propane waits in the tank for years without stabilizer or rotation.

Quick recommendations

• A tank with a gauge, about $69.99: the Flame King YSN230 20-LB Steel Propane Tank with… holds the full 20 pounds and its built-in gauge shows where the tank stands before the forecast turns.
• One-room outage heat, about $89.97: the Mr. Heater Portable Buddy Propane Heater (MH9BX) runs 4,000 or 9,000 BTU with an oxygen-depletion shutoff, which is what makes a 20-pound tank last for days.
• The hose that connects them: the Mr. Heater 10 ft Propane Hose Assembly runs the Buddy from a 5 to 20-pound tank instead of one-pound canisters, with no fuel filter needed (the listing drifts in and out of stock, so check the current price).

Know the BTU rating, do the one division, and fill the tank before the weather turns.