How Long Does a Propane Tank Last?

There are two completely different questions hiding inside this one, and mixing them up is why the answers you find online contradict each other so badly.

The first is about storage: does the propane sitting in that tank go bad while you are not using it? The second is about runtime: how many hours will this tank actually feed the thing you want to run? One has a one word answer. The other is arithmetic, and once you have seen the arithmetic you can work out any tank against any appliance in about ten seconds without looking anything up again.

Quick Answer

Propane itself does not expire. Stored in a sealed steel cylinder it stays chemically identical for decades, which is the single biggest advantage it has over gasoline. What has a date is the cylinder, not the fuel: a standard portable propane tank in the United States is certified for 12 years from its manufacture date, then has to be requalified before anyone will legally refill it. On runtime, the usable number is a full 20 lb tank holds about 4.7 gallons, or roughly 430,000 BTU. Divide that by your appliance's BTU per hour rating and you have your answer: about 8 to 14 hours on a gas grill, roughly 10 hours on a patio heater, 48 to 100 hours on a small radiant heater, and about 5 to 6 hours on a mid size dual fuel generator at half load.

Key Takeaways

  • The fuel is effectively immortal. The tank is not. Propane has no shelf life worth planning around. The 12 year certification stamp on the collar is the only clock that actually runs, and an out of date cylinder is one that no filling station is permitted to touch.
  • Runtime is one division problem. Tank BTU divided by appliance BTU per hour equals hours. A 20 lb tank is about 430,000 BTU. A 1 lb camping cylinder is about 21,500. Everything else scales from there.
  • A tank is only ever 80 percent full, and that is deliberate. Liquid propane expands sharply with temperature, far more than water does, so the remaining fifth of the tank is vapor space that keeps a hot afternoon from turning into a hydraulic problem. A "full" 20 lb tank containing 20 lb of propane is the correct fill.
  • Cold weather does not shorten the fuel's life, it shortens the tank's delivery rate. Propane has to boil off the liquid surface to reach the burner. A small cylinder that is nearly empty on a freezing night cannot vaporize fast enough to feed a large appliance, and the burner starves even though there is fuel in the tank. This is the failure people misread as a bad regulator.
  • Weighing beats guessing. The tare weight is stamped on the collar as "TW". Subtract it from what your bathroom scale reads and you have pounds of propane remaining, which is a real number rather than an estimate from a gauge that mostly reports temperature.
  • Storage is outdoors, upright, always. Propane vapor is heavier than air and pools at floor level. A cylinder in a basement or an attached garage is a different category of risk from one on a patio.

Why Propane Does Not Go Bad

Gasoline degrades because it is a blend of dozens of hydrocarbons that oxidize, evaporate, and, in ethanol blends, absorb water out of the air. All three of those failure modes need something the fuel is exposed to. Propane in a sealed cylinder is exposed to nothing.

It is also a single simple molecule rather than a cocktail, so there is no light fraction to flash off and no heavy fraction left behind. There is no ethanol, so there is nothing hygroscopic to pull moisture in. And the container is a sealed pressure vessel by necessity, not a vented can, so there is no daily breathing cycle bringing humid air in and out. A cylinder filled in 2015 and stored in a shed will run your grill in 2035 exactly as well as it would have on the day it was filled.

This is the whole reason propane keeps showing up in serious backup power planning. The comparison against stored gasoline is not close: three months untreated and about a year stabilized, covered in detail in How Long Does Gasoline Last?, against a fuel that genuinely does not care how long it has been sitting. The tradeoff is energy density and cost per BTU, not longevity.

The honest caveat is that the cylinder is a steel object outdoors. Rust at the base, a corroded valve, a damaged foot ring, or a collar too pitted to read are all real, and all of them are reasons a filling station will refuse a tank regardless of what the date says.

The Only Date That Matters: Requalification

Look at the collar, the metal handle ring at the top. Stamped into it is a date in month and year form, sometimes with a letter after it. That is the manufacture date, and under US Department of Transportation rules a cylinder is good for 12 years from that date. After that it has to be requalified by an authorized facility before it can be legally refilled.

Once requalified, how long you get depends on the method used, and the letter stamped after the new date tells you which one:

  • No letter, or a volumetric expansion test, generally buys another 12 years.
  • "E" for external visual inspection buys 5 years. This is the fast, cheap, common one, which is why so many requalified tanks come back due again surprisingly soon.
  • "S" for proof pressure test buys 10 years.

In practice, most people never requalify a 20 lb grill tank. The exchange cage outside the hardware store is cheaper than the test, and swapping an expired tank into an exchange program is the normal way these leave circulation. That is fine, with one caveat worth knowing: exchange tanks are usually filled to 15 lb rather than 20, so an exchange is about three quarters of a refill by volume. If you are budgeting runtime for an outage, refill rather than exchange.

⚠️ SAFETY: It is illegal to fill a DOT cylinder past its certification date, and a filling station that agrees to anyway is not doing you a favor. Check the collar date before you plan around a tank. Verify current requalification requirements against PHMSA guidance and 49 CFR 180.209.

The Arithmetic: Tank Size to Hours

Propane carries roughly 91,500 BTU per gallon, and liquid propane weighs about 4.24 pounds per gallon. Those two numbers generate the entire table below, and they are the only two worth memorizing.

Tank Propane when full Approximate energy Typical use
1 lb cylinder (16.4 oz) 0.24 gallons ~21,500 BTU Camp stoves, small radiant heaters, lanterns
15 lb 3.5 gallons ~324,000 BTU What most exchange cages actually give you
20 lb (the standard grill tank) 4.7 gallons ~430,000 BTU Grills, patio heaters, fire pits, portable generators
30 lb 7.1 gallons ~645,000 BTU RVs and campers, usually in pairs
40 lb 9.4 gallons ~860,000 BTU Larger RVs, construction heaters
100 lb 23.6 gallons ~2.16 million BTU Serious home backup, standby generator feed
250 gallon (200 usable) 200 gallons ~18.3 million BTU Supplemental home heat, water heating
500 gallon (400 usable) 400 gallons ~36.6 million BTU Whole home heat plus standby generator

Note what happens to the big tanks. A 500 gallon tank never holds 500 gallons. Rated capacity is the water capacity of the vessel, and the working fill is 80 percent of it, so the honest number is 400.

Runtime by Appliance

Take the tank energy from the table above and divide by the appliance's rating, which is printed on its data plate. A 20 lb tank at 430,000 BTU produces the following.

Appliance Typical draw Hours on a 20 lb tank
Gas grill, three burner, medium to high 30,000 to 55,000 BTU/hr 8 to 14 hours of active cooking
Patio heater, tower style 40,000 BTU/hr About 10 hours
Propane fire pit 50,000 BTU/hr About 8 hours
Portable radiant heater, low setting 4,000 BTU/hr About 100 hours
Portable radiant heater, high setting 9,000 BTU/hr About 48 hours
Two burner camp stove, one burner running 10,000 BTU/hr About 43 hours
Dual fuel generator, mid size, half load ~0.8 gallons/hr 5 to 6 hours
Tankless water heater, running 120,000 to 180,000 BTU/hr 2.5 to 3.5 hours of actual flow

The generator row is the one that reorganizes people's plans. A 20 lb tank is most of a night, not a weekend. Running a generator through a multi day outage on propane means either a 100 lb cylinder, a bank of 20 lb tanks with a changeover regulator, or a permanent tank. If you have not settled the sizing question yet, What Size Generator Do I Need? works through load math first, and Generator vs. Solar: What Actually Keeps the Lights On covers the case for skipping fuel entirely.

The Cold Weather Problem Nobody Warns You About

This is the part that catches people out, and it is the reason a propane heater that worked fine in October sputters in January.

Your appliance burns propane vapor, but the tank stores it as a liquid under pressure. The liquid has to boil off its own surface continuously to keep the vapor space supplied, and boiling takes heat, which the liquid pulls from the tank wall and ultimately from the outside air. Two things therefore limit how fast a tank can deliver fuel: the temperature of the air, and how much liquid surface area is in contact with the tank wall.

Both work against you at once in winter. Cold air slows the boil, and as the tank empties the liquid level drops, shrinking the wetted wall area doing the heat transfer. A full 20 lb cylinder on a mild day can comfortably sustain a large burner. The same cylinder at a quarter full on a freezing night may not sustain half that, and the symptom is a burner that lights, runs, then fades. Frost forming in a band around the outside of the tank is the visible tell: that band marks the liquid level, and it means the tank is absorbing heat faster than the air can replace it.

Propane stops vaporizing altogether at about 44 degrees below zero Fahrenheit, its boiling point at atmospheric pressure, though real problems start long before that. The practical fixes are all about surface area and mass, not about the fuel:

  • Use a bigger tank than the appliance strictly needs in cold climates. A 100 lb cylinder feeding a heater sized for a 20 lb one will never starve it.
  • Manifold two tanks together rather than running one hard. Two cylinders share the draw and each stays warmer.
  • Keep tanks fuller through winter, refilling at a third rather than running to empty, because the last third is where delivery rate collapses.
  • Never warm a tank with a flame, a heat gun, or by bringing it indoors. Direct heat on a pressure vessel is exactly the wrong instinct.
⚠️ SAFETY: Never apply direct heat to a propane cylinder to improve its flow rate, and never move a cylinder indoors to warm it. Both create a pressure hazard and, indoors, a vapor accumulation hazard. If a tank cannot keep up, the answer is a larger tank or a second one, never added heat. Verify handling requirements against NFPA 58 and your supplier's guidance.

How Much Is Left? Three Ways, One of Them Reliable

Weigh it. Find "TW" stamped on the collar, which is the tare weight of the empty cylinder in pounds. A typical 20 lb tank reads somewhere around 17 or 18. Put the tank on a bathroom scale, subtract the tare, and what remains is pounds of propane. Multiply by roughly 21,500 BTU per pound to convert to runtime. This is the only method that gives you a real number.

The warm water trick. Pour a cup or two of warm water down one side of the tank and run your hand down the same path. The propane inside absorbs heat, so the wetted metal below the liquid line feels distinctly cooler. Where the temperature changes is your fuel level. Free, surprisingly accurate for a rough read, and it works in the dark.

The dial gauge. The inline gauges that screw between tank and hose read pressure, and propane vapor pressure is dominated by temperature rather than by how much liquid is left. The needle will happily sit in the green on a warm day with almost nothing in the tank, then read low on a cold morning with the tank most of the way full. Treat these as a leak indicator, not a fuel gauge.

Storing Cylinders Without Creating a Hazard

Everything above is about getting fuel out of the tank. This part is about the tank not becoming the emergency.

⚠️ SAFETY: Propane vapor is heavier than air and flows downhill, pooling in basements, pits, and floor level spaces where a pilot light, furnace, or electrical switch can ignite it. Store cylinders outdoors, upright, in a ventilated space away from the home, never in a basement, never in a living space, and never inside a vehicle for longer than the drive home. Verify quantity and placement limits against NFPA 58 and your local fire code, which is frequently stricter.
  • Outdoors and upright. Upright keeps the pressure relief valve in the vapor space where it can vent gas rather than liquid. A cylinder on its side is a cylinder whose safety valve no longer does what it was designed to do.
  • Shaded, off bare ground. A pallet, a paving slab, or a purpose built stand keeps the base out of standing water, which is where the rust that condemns a tank starts.
  • Valve closed, plug or cap fitted whenever a tank is not connected, including on a tank you think is empty. An empty tank still holds vapor.
  • Not in the attached garage. The US Fire Administration's guidance on flammable materials points to a detached shed, and propane's tendency to pool at floor level makes the attached garage a particularly poor choice.
  • Soapy water finds leaks. Brush the valve and connections with a dish soap solution and open the valve. Growing bubbles mean a leak. Never use a flame to check, which should not need saying and does.
  • Transport upright and secured, windows open, and take it straight home. A cylinder rolling loose in a hot trunk is a genuinely bad idea.

Burning It Indoors: The Line That Matters

Propane burns cleanly, and "cleanly" gets misread as "safely indoors." Complete combustion produces carbon dioxide and water vapor. Incomplete combustion, which is what happens when a burner is starved of oxygen in a closed room, produces carbon monoxide, and carbon monoxide is odorless.

⚠️ SAFETY: Never operate a propane grill, camp stove, patio heater, or construction heater inside a home, tent, garage, or any enclosed space. Only appliances specifically rated for indoor use, fitted with an oxygen depletion sensor, and operated with the ventilation their manual specifies belong inside. Install working carbon monoxide alarms on every level of the home and outside sleeping areas. Verify against CPSC and CDC carbon monoxide guidance.

The small radiant heaters sold as indoor safe do carry an oxygen depletion sensor and are genuinely designed for the job, and they still specify a minimum room volume and a cracked window. Read the plate. The heater that ran fine in an open garage last winter is not the same situation as the same heater in a sealed bedroom, and the difference is not detectable by smell. The same reasoning applies to every fuel burning option covered in How to Stay Warm Without Power.

How Much to Keep on Hand

Work backwards from a scenario rather than forwards from a shopping list.

  1. Pick the job. Cooking only is a very different quantity from heating one room, which is different again from running a generator.
  2. Find the BTU per hour on the appliance data plate, and be honest about hours per day. A heater on low for eight overnight hours is 8 times its low rating, not its high one.
  3. Multiply by the days you are planning for, then divide by 430,000 to get tanks.

Worked example: a radiant heater on low at 4,000 BTU/hr, eight hours a night, over a five day winter outage is 160,000 BTU. That is well under half of one 20 lb tank. Cooking two meals a day on a single camp stove burner over the same five days adds maybe 20,000 BTU. Two tanks covers heat and food for five days with a wide margin, and most households already own one.

Now the same five days with a generator at half load: about 0.8 gallons an hour, and even running it only eight hours a day is 32 gallons, or roughly seven 20 lb tanks. That is the number that tells you whether your backup power plan is a cylinder plan or a permanent tank plan, and it is better to learn it now than on day two.

FAQ

How long does propane last in storage?

Indefinitely. Propane is a single stable molecule in a sealed pressure vessel, with nothing to oxidize, evaporate off, or absorb water. Fuel bought a decade ago burns identically today. The limit is the cylinder's 12 year certification and its physical condition, not the propane.

How long does a 20 lb propane tank last?

About 430,000 BTU worth, which works out to roughly 8 to 14 hours of grilling, about 10 hours on a patio heater, 48 to 100 hours on a small radiant heater depending on the setting, and 5 to 6 hours on a mid size dual fuel generator at half load. For most households a grill tank is a full season of cooking and a single night of generator running.

How long does a 1 lb propane tank last?

A 1 lb camping cylinder holds about 21,500 BTU. On a 10,000 BTU/hr camp stove burner that is roughly two hours of active cooking. On a small radiant heater at its 4,000 BTU/hr low setting it is about five hours, which is why a single overnight in cold weather usually eats one cylinder. The 16.4 oz green canisters and the 1 lb figure refer to the same thing.

How long does a 100 lb propane tank last?

About 2.16 million BTU. On a mid size generator at half load that is roughly 29 hours of continuous running, or three to four days at eight hours a day. On a radiant heater on low it is measured in weeks. This is the size most people land on once they have done the generator arithmetic on a 20 lb tank.

How long does a 500 gallon propane tank last?

A 500 gallon tank holds 400 usable gallons, around 36.6 million BTU. For a home heated entirely with propane, that is most of a winter in a mild climate and considerably less in a cold one, since whole home propane heating typically runs somewhere between 500 and 1,200 gallons a year. If a standby generator shares the same tank, plan for it to draw down that reserve fast: two to three gallons an hour under load adds up to more in three days than a week of ordinary heating.

How long does a propane tank last on a grill?

Take your grill's total BTU per hour rating from its data plate and divide 430,000 by it. Most three burner grills land between 30,000 and 55,000 BTU per hour on high, giving 8 to 14 hours of burner time. Since a typical cookout is an hour or less and rarely uses every burner on high, that is commonly 15 to 20 sessions from one tank.

How long does a propane tank last on a fire pit?

Fire pits run hot for their size, typically 50,000 BTU per hour, so about 8 hours from a 20 lb tank. Turning it down materially extends that, since consumption tracks the flame directly. If you are running one regularly, a 100 lb cylinder with an extension hose costs less per hour and saves the swap.

How long does a propane heater last?

Depends entirely on the setting, which is the useful part. A portable radiant heater drawing 4,000 BTU per hour on low runs about 100 hours from a 20 lb tank, while the same heater on high at 9,000 BTU per hour runs about 48. Doubling the output roughly halves the runtime, so heating a small closed off room on low is dramatically more efficient than heating a large one on high.

Does propane go bad if the tank sits unused for years?

The propane does not. The tank might. A cylinder that has sat outdoors for years can develop rust at the foot ring, a corroded or stiff valve, or a collar too pitted to read the date on, and any of those is grounds for a filling station to refuse it. Check the certification date, look hard at the base and valve, and brush the connections with soapy water before trusting an old tank with anything important.

Why does my propane tank feel frozen or frosty while running?

That frost band marks the liquid level, and it means the tank is boiling off fuel faster than the surrounding air can supply the heat to do it. It is normal at high draw and a warning sign in cold weather, because delivery rate falls as the tank cools and empties. Reduce the draw, switch to a larger tank, or manifold a second cylinder in. Do not apply heat.

Can I keep a propane tank in the garage over winter?

Storing it outside is the right answer, and cold weather is not the reason to bring it in. Propane is completely unbothered by winter temperatures in storage, while an attached garage is exactly the kind of enclosed, floor level space where a slow leak accumulates near ignition sources. Keep cylinders outdoors, upright, and shaded, and check your local fire code, which may set stricter limits than the national standard.

The Bottom Line

Propane does not expire, which makes it the easiest fuel to store and the hardest to plan runtime for, because all the difficulty moves from the calendar to the arithmetic. Remember two numbers, 430,000 BTU in a 20 lb tank and 91,500 BTU per gallon, and you can size any tank against any appliance on the spot. Check the collar date before you count on a cylinder, weigh it instead of trusting the gauge, keep it outdoors and upright, and never burn it indoors unless the appliance was specifically built for it.

The propane in your tank will outlast the tank, and probably outlast the grill. The only question worth asking is whether you have enough of it for the job you are actually planning for. Go weigh the tank. Be prepared. Stay savage.

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