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A standard 20 lb propane tank, the same size used for a gas grill, holds roughly 20 times the fuel of a 1 lb disposable cylinder. Here is the formula behind its run time, and a table across the BTU tiers a hose-connected tank actually feeds.
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The Formula
A 20 lb tank holds roughly 432,000 BTU of usable energy — that’s 20 lb multiplied by the standard 21,600 BTU-per-pound conversion. Divide that figure by your heater’s BTU rating to get an estimated run time in hours. A 9,000 BTU heater works out to roughly 48 hours. A 125,000 BTU forced-air heater works out to roughly 3.5 hours.
Run Time by BTU Rating
- 9,000 BTU (portable radiant): roughly 48 hours
- 18,000 BTU (portable radiant, high): roughly 24 hours
- 30,000 BTU (permanent vent-free): roughly 14.4 hours
- 45,000 BTU (tank-top radiant): roughly 9.6 hours
- 60,000 BTU (forced-air): roughly 7.2 hours
- 125,000 BTU (forced-air, full output): roughly 3.5 hours
These are full-burn estimates at maximum output. Real-world run time is usually a little longer, since most heaters cycle rather than running at their highest setting the entire time. For the Mr. Heater Buddy lineup specifically, see our Buddy-specific run time table.
Why This Table Works Across Brands
The 21,600 BTU-per-pound conversion is a property of propane, not a brand-specific number. Any heater hose-connected to a 20 lb tank follows the same math, regardless of manufacturer. If your heater’s BTU rating isn’t listed above, divide 432,000 by your heater’s specific output for your own estimate.
Refillable vs. Disposable: Why 20 lb Tanks Make Sense for Regular Use
For occasional use, 1 lb disposable cylinders are simple. For a heater run regularly through a season, a refillable 20 lb tank costs less per hour of use and means far fewer trips to buy fuel. A Flame King 20 lb steel tank with a working OPD valve, paired with a regulator hose with a gauge, is a common starting setup for connecting a portable heater to a larger tank. Our tanks, hoses and accessories guide covers the fittings and adapters this switch requires.
Checking Remaining Fuel on a 20 lb Tank
Unlike a 1 lb disposable cylinder, a 20 lb tank can use an inline pressure gauge, or a simple weight check. A full 20 lb tank weighs roughly 37 to 38 lb including the tank’s own weight, which is usually stamped on the collar. Either method tracks remaining fuel more precisely than guessing.
Running Two Heaters Off One Tank
Some setups draw two heaters from the same 20 lb tank through a manifold or splitter — a Big Buddy plus a smaller heater in a large wall tent is one example. This shortens total run time in proportion to the combined BTU draw of both heaters, not just the larger one. If you run more than one heater off a single tank, divide the tank’s total capacity across the combined output of both, rather than treating each heater’s run time as independent of the other.
Signs a Tank Is Close to Empty
A 20 lb tank doesn’t give a clean warning before it runs dry, but a few signs are worth watching for. A noticeably weaker flame, especially in colder weather, can mean the tank’s internal pressure is dropping as it empties, since cold reduces a nearly-empty tank’s pressure more than a full one. Frost forming on the tank’s exterior during use is another sign, caused by the rapid pressure drop as the last of the liquid propane vaporizes. Neither sign is a precise gauge, which is exactly why a dedicated pressure gauge or a periodic weight check, covered above, is worth adding if you rely on a tank regularly.
Running Cost at This Scale
Refilling a 20 lb tank is typically cheaper per gallon than exchanging one at a hardware store or gas station, since exchange pricing includes a convenience premium. For anyone running a heater regularly through a cold season, finding a local refill station rather than defaulting to exchange adds up to meaningful savings. Our running cost calculator guide covers converting these run-time figures into an actual dollar cost.
Why a 20 lb Tank Needs Periodic Recertification
Every propane tank carries a manufacture date stamped on its collar, and most need recertification every 5 or 10 years depending on that date and the tank’s construction. Refill stations generally won’t fill a tank past its recertification date until it’s either recertified or replaced. This isn’t specific to any one tank brand — it’s a standard safety requirement across the category. Checking the stamped date before heading to a refill station saves a wasted trip if the tank has quietly aged out of service without you noticing.
Matching Tank Size to Actual Usage
A 20 lb tank is the standard mid-size choice, but it isn’t the right fit for everyone. Someone running a high-output forced-air heater for hours at a stretch, several days a week through a whole winter, may find a 30 lb, 40 lb, or even 100 lb tank cuts refill trips down meaningfully, at the cost of more size and weight to store and move. On the other end, someone using a portable radiant heater a handful of times a season may find 1 lb disposable cylinders simpler than managing a 20 lb tank at all. Match the tank to how often and how hard you actually run the heater, not to whichever size happens to be most common on the shelf.
Transporting a 20 lb Tank Safely
A 20 lb tank should always ride upright in a vehicle, secured so it can’t tip or roll, and never in a fully sealed trunk on a hot day if it can be avoided. Most vehicles with an open bed or a well-ventilated cargo area handle this better than a sealed sedan trunk. If a tank must travel in an enclosed space, cracking a window slightly and keeping the trip short reduces the already-small risk of heat buildup around the valve. This is standard guidance for any propane cylinder, not something specific to a particular tank brand or heater model.
Propane Run-Time Calculator
Estimate how long a tank will last. Real-world run time varies with weather, altitude and burner setting.