Propane Tank Calculator

How long a propane tank lasts, in one equation

Propane carries about 21,500 BTU of usable heat per pound. That single constant answers the whole question. Multiply the pounds in your tank by 21,500 to get the energy you are holding, then divide by how fast your burner eats it. A standard 20 lb grill tank is 20 x 21,500 = 430,000 BTU. A 30 lb tank is 645,000, a 40 lb is 860,000, and a 100 lb cylinder holds 2,150,000 BTU.

The catch is the second number. Appliances are labelled with their maximum rating, and almost nobody runs at maximum. A grill sitting at medium is drawing roughly three quarters of its plate rating, which is why this calculator asks how hard you run it rather than assuming full blast.

Worked example: a 20 lb tank on a gas grill

A three-burner grill rated at 30,000 BTU/hr, run at a typical 75%, consumes 22,500 BTU per hour. 430,000 / 22,500 = 19.1 hours, or about 19 h 7 min of cooking. At roughly three hours per cookout — preheat, sear, rest, a second round for the neighbours — that is six good grilling sessions from one tank. If you paid $22 for the tank, your grilling costs about .15 an hour.

Worked example: a patio heater on a cold evening

Patio heaters have no low setting worth speaking of; people run them wide open at 40,000 BTU/hr. 430,000 / 40,000 = 10.75 hours, or 10 h 45 min. Three or four evenings on the deck and the tank is done, which is why heaters are the appliance that surprises people most. At $22 per tank that is $2.05 an hour of warmth.

Worked example: a generator during an outage

A dual-fuel portable generator at half load draws in the region of 56,000 BTU/hr. A 20 lb tank gives 430,000 / 56,000 = 7.7 hours — one overnight, no more. Anyone planning to ride out a multi-day outage on propane needs three or four tanks, or a 100 lb cylinder that stretches the same load to about 38 hours.

Checking the level without a gauge

The warm-water trick is the field method. Fill a jug with hot tap water, pour it slowly down one side of the tank, then run your hand down the same stripe. Above the liquid line the steel stays warm; below it the propane absorbs the heat and the metal turns noticeably cold. The boundary you feel is the fill level, accurate to an inch or so.

For a real number, use a bathroom scale. Stamped around the collar of every cylinder is TW, the tare weight — the weight of the empty tank, typically 17 to 18 lb on a 20 lb grill bottle. Weigh the tank, subtract the tare, and the remainder is propane in pounds. A tank reading 27 lb with a TW of 17.5 holds 9.5 lb, which is 204,000 BTU: enough for four more hours of grilling at medium heat, not enough for a long smoke.

Why cold weather steals your run time

Propane leaves the tank as vapour, and vapour only forms as fast as the liquid can boil. Boiling depends on temperature. At 70°F a full 20 lb tank can sustain roughly 55,000 to 65,000 BTU/hr indefinitely; near 0°F that vaporisation rate collapses toward 25,000 BTU/hr, and it falls further as the tank empties because there is less wetted surface area inside. The gas is still in there — the tank simply cannot deliver it fast enough. A heater or a generator that runs perfectly in August will sputter and drop out in January with a half-full tank. The fixes are a bigger cylinder, two tanks manifolded together, or keeping the tank out of the wind.

Exchange versus refill: the actual math

Exchange cages are convenient and expensive. Most fill to 15 lb rather than 20, and the sticker says so in small print. Paying $22 for 15 lb is .47 per pound, or $6.82 per 100,000 BTU. A refill station charges by the gallon — propane runs about 4.24 lb per gallon, so a 20 lb tank takes roughly 4.7 gallons — and at $4.00 a gallon that is 8.80 for the full 430,000 BTU, or $4.37 per 100,000 BTU. Refilling comes out about 36% cheaper for the same heat, and you keep your own tank.

Exchange wins in exactly one situation: your cylinder is out of date. US tanks carry a manufacture date stamp and are good for 12 years, then need re-certification. Hand a lapsed tank to an exchange cage and you walk away with a certified one at no extra charge.

What this estimate cannot see

Burn time is calculated from steady-state consumption. Real appliances cycle: a thermostatically controlled RV furnace or a grill with the lid down runs its burner only part of the time, which stretches results well beyond the number here. Wind blowing across a grill, high altitude, and a partly clogged regulator all push the other way. Treat the output as the honest floor for continuous burn and expect real life to land somewhat above it.