🔥 Propane Tank Duration Calculator

What This Calculator Does

This calculator answers the cookout question: how long will this tank last? Choose the tank size — the ubiquitous 20 lb grill cylinder up to 100 lb heater tanks — enter your appliance's BTU-per-hour rating (printed on its label or in the manual), and it returns the burn time at full output, with the honest note that real use at partial burner settings stretches it further.

Propane anxiety is a fixture of American grilling precisely because tanks are opaque and gauges are rare. The energy arithmetic is actually simple, and pairing it with the weighing trick in the guidance below replaces guesswork with numbers.

The Energy Math in a Tank

Propane's energy density does the heavy lifting: about 91,500 BTU per gallon, roughly 21,500 per pound. A “20 lb” cylinder properly filled holds about 4.7 gallons (the 80% fill rule leaves expansion space), for a total near 430,000 BTU. Burn time is total BTU ÷ appliance BTU/hr: the classic 30,000 BTU three-burner grill runs about 14.3 hours flat out; a 60,000 BTU patio heater, 7 hours; a 10,000 BTU camp stove burner, 43. Larger tanks scale linearly — a 100 lb cylinder carries about 2.15 million BTU.

The full-output figure is the conservative floor, because appliances rarely run wide open: a grill preheats at full and cooks at half, roughly doubling its real-world hours, and a heater on its middle setting does the same. The honest way to use the number is as a floor for planning (“the party needs 5 hours of heater — one tank is comfortably enough”) rather than a countdown clock, and to apply your own duty-cycle judgment on top.

Knowing What Is Left

The weighing method is the accurate one. Every cylinder's collar is stamped with its tare weight — “TW 17” meaning the empty tank weighs 17 lbs — so a bathroom-scale reading of 25 lbs on a TW-17 tank means 8 lbs of propane remain: about 172,000 BTU, or 5.7 hours of a 30,000 BTU grill. This thirty-second check before hosting is the single best cure for mid-cookout flameouts, and it is exact in a way no gauge accessory quite matches.

The folk method works too: pour warm water down the tank's side and run a hand down after — the metal feels distinctly cool at and below the liquid propane line (the liquid absorbs the heat; the vapor space above does not). Inline pressure gauges, by contrast, mislead by design: tank pressure depends on temperature, not fill level, staying essentially constant until the tank is nearly empty — a gauge reading “full” describes the weather more than the fuel. Weight tells the truth; keep a cheap luggage scale by the grill and the mystery ends.

Cold Weather, Exchanges, and Tank Care

Winter changes propane behavior in a way worth understanding. Appliances burn propane vapor, which the liquid must continuously boil off — and vaporization slows as temperature falls, dropping tank pressure. A cold, quarter-full 20 lb tank may be unable to feed a 60,000 BTU heater even though fuel remains: the flame starves and sputters, a pressure limit rather than an empty tank. The mitigations follow from the physics: keep winter tanks fuller (more liquid surface to vaporize), shelter them from wind, never heat a tank with flame, and size up the cylinder for big heaters in cold climates.

Buying has its own arithmetic: exchange programs famously fill to 15 lbs rather than 20, trading a quarter of the capacity for convenience, while a refill station charges by the gallon into your own tank — cheaper per BTU nearly always, and it keeps a good tank in service (cylinders require requalification 12 years from their stamped manufacture date). Store tanks outdoors, upright, valve closed; transport upright and ventilated. None of it is complicated — like the burn-time math itself, it is a handful of numbers standing in for a lot of guessing.

Quick Tips

• 20 lb tank ≈ 430,000 BTU; hours = 430,000 ÷ appliance BTU/hr at full blast.
• Weigh it: scale reading − stamped TW = pounds of propane left (× 21,500 BTU).
• Pressure gauges track temperature, not fill — trust the scale.
• Cold tanks lose pressure: keep them fuller and sheltered in winter.

Frequently Asked Questions

How long does a 20 lb propane tank last?

A full 20 lb tank holds about 430,000 BTU. Divide by the appliance's BTU-per-hour rating: a 30,000 BTU/hr grill runs about 14.3 hours; a 60,000 BTU/hr patio heater about 7. Burners rarely run at full blast, so real-world times run longer.

How many BTUs are in propane?

About 91,500 BTU per gallon, and a pound of propane is roughly 21,500 BTU. A '20 lb' tank actually holds about 4.7 gallons when properly filled (tanks fill to 80% for safety expansion room).

How can I check how much propane is left?

Weigh the tank: subtract the tare weight (the 'TW' stamped on the collar, usually ~17 lbs for a 20 lb tank) from the scale reading — the remainder is propane pounds. The warm-water trick also works: pour hot water down the side; the tank feels cool at the liquid level.

Why does my tank seem to die early in winter?

Cold reduces tank pressure — propane vaporizes more slowly as temperature drops — so high-BTU appliances can starve on a cold, low tank even with fuel remaining. Keeping tanks fuller in winter and out of wind helps; it is a pressure issue, not missing fuel.

Cookout Math, Free to Use

This calculator runs entirely in your browser, dividing tank energy by burner appetite the moment you type. It needs no account or download and works on any device. Like every tool here, it is free to use as often as you like.

Related Tools

For related numbers, the BTU calculator sizes heating for spaces, the time converter handles the hours, and the liters to gallons converter switches fuel volumes.