Propane Tank Life Calculator

How long a propane tank lasts, in one formula

A propane tank has no clock on it. It has a weight of fuel and a burner with a rating, and the run time falls out of those two numbers. The formula is the tank capacity in pounds, times how full the tank is, times the heat in a pound of propane, divided by the BTU per hour the appliance burns.

One pound of liquid propane releases about 21,548 BTU when it burns. A gallon of propane holds 91,500 BTU and weighs about 4.24 pounds, so the two figures agree once you divide one by the other. Nothing here depends on the shape of the tank or the brand on the collar.

Hours = pounds of propane in the tank x 21,548 / appliance BTU per hour

That is the whole calculation. The rest of this page is about picking honest numbers for the three parts: the pounds, the fill level, and the BTU rating on the appliance. Get those right and the answer lands within a few minutes of what you will see in real use.

The one thing the formula ignores is how fast the tank can turn liquid into vapor. A small tank can only feed so many BTU per hour, no matter what the division says. That limit gets its own section further down, because it is the reason a 20 lb cylinder cannot run a big tankless water heater.

What a 20 lb tank holds in BTU

The 20 lb tank is the one most people own, the squat cylinder that lives under a grill. Filled to the legal limit it holds 20 pounds of propane, which is 4.7 gallons and about 430,000 BTU. That single figure answers most of the questions people ask about tank life.

The 20 lb name refers to the propane, not to the steel. A 20 lb cylinder has a water capacity of about 47.6 pounds, and NFPA 58 caps the fill at 80 percent of that water capacity. Eighty percent of 47.6 pounds of water is about 4.6 gallons of space, and 4.6 gallons of propane weighs roughly 20 pounds. That is not a coincidence. The 80 percent rule exists because propane expands as it warms and the tank needs headspace for the liquid to grow into. A cylinder filled to the brim on a cool morning will vent propane by the afternoon.

The same arithmetic covers the other sizes. A 5 lb camp cylinder holds 107,740 BTU, a 30 lb tank holds 646,440 BTU, a 40 lb tank holds 861,920 BTU, and a 100 lb tank holds 2,154,800 BTU. Doubling the tank doubles the heat, which is the only part of this that scales in a straight line. The EIA glossary entry for propane lists the heat content per pound and per gallon if you want the source figures.

Propane is also sold by the gallon, and the two units get mixed up on price signs. Divide pounds by 4.24 to get gallons. A 20 lb tank is 4.7 gallons, so a refill priced at $3.50 a gallon costs about $16.50 for a full tank. When a station quotes a price per gallon, that is what the pump meter is counting.

Worked example: a 30,000 BTU grill

A three burner gas grill is usually rated between 24,000 and 36,000 BTU per hour. Take 30,000 as the middle of that range. A full 20 lb tank holds 430,960 BTU.

430,960 / 30,000 = 14.4 hours

So one tank covers about 14 hours of grilling with all burners on high. A two hour cookout uses 14 percent of a tank, which means seven cookouts empty it. A household that grills twice a week gets about three and a half weeks from a refill, so a summer of weekend cooking takes two or three tanks rather than one. The tank that dies in the middle of a holiday dinner is usually the third one, not the first.

Turn a burner off and the number moves. Running one 10,000 BTU burner instead of all three drops the draw to a third, and the same tank stretches to 43.1 hours. Most cooking is not done at full blast, so real tanks last longer than the headline figure. Preheat and searing are the expensive minutes.

Cost follows the same math. At $3.50 a gallon, 30,000 BTU per hour burns 0.328 gallons, which is $1.15 an hour. A two hour cookout costs about $2.30 in propane, and the whole tank costs $16.51. Compare that with a few dollars of charcoal and the running cost is competitive, though the tank itself is the expensive part.

One more check on the arithmetic: 14.4 hours times 30,000 BTU is 432,000 BTU, and the tank holds 430,960. The small gap is rounding, not a mistake.

Tank sizes side by side

Every size below uses the same 21,548 BTU per pound, so the columns move together. The hours column assumes a 30,000 BTU grill running at full setting.

The jump from 20 lb to 100 lb is the one that changes how people use propane. A 100 lb cylinder holds five times the fuel of a grill tank and runs a 30,000 BTU grill for 71.8 hours, which is a whole season of cooking. The catch is weight: a full 100 lb cylinder is about 170 pounds with the steel, so it stays on a dolly or sits on a pad next to the house and gets filled by a service truck.

Above 100 lb the tanks stop being portable. A 250 gallon house tank holds 1,000 pounds of propane, or 21.5 million BTU, and it is filled by a truck on a schedule rather than carried to a station. The formula does not change, but the fill level does: those tanks are usually kept between 20 and 80 percent so there is always a buffer for a cold snap.

Grills and heaters burn at different rates

This is where most estimates go wrong. The tank is the same, but a patio heater pulls 48,000 BTU per hour while a camp stove pulls 10,000, and the run time differs by a factor of five.

A patio heater is the hungriest thing most households own. At 48,000 BTU per hour it empties a 20 lb tank in 9.0 hours, which is three or four evenings on the deck. The same tank on a three burner grill gives 14.4 hours, and on an RV furnace at 20,000 BTU it gives 21.5 hours of continuous heat. People who run a furnace overnight on a 20 lb tank burn through a tank every couple of nights in cold weather, which is why RV owners carry 30 lb or 40 lb cylinders instead.

The fire pit number surprises people too. A 60,000 BTU fire pit runs 7.2 hours on a full 20 lb tank, so a three hour evening with friends uses nearly half a tank. A fire pit with a low setting at 25,000 BTU more than doubles that, and the flame still looks right in the dark.

The practical rule: read the BTU rating on the label before you buy the appliance. A 30,000 BTU grill and a 48,000 BTU heater look similar on the showroom floor and behave nothing alike at the tank.

Fill level: why the gauge lies about what is left

The calculator asks how full the tank is because a tank that feels heavy is not necessarily a tank that is full. The percentage you enter multiplies the capacity directly, so a half full 20 lb tank behaves exactly like a 10 lb tank.

Two things make the fill level hard to judge. The first is the gauge. Most grill tanks have no gauge at all, and the ones that do are usually a pressure gauge on the regulator, which reads the same until the tank is nearly empty. Pressure stays steady while liquid propane boils off, so a gauge that tracks pressure tells you almost nothing about how much is left.

The second is the exchange rack. A swap tank from a grocery store or a hardware store is typically filled to 15 pounds, not 20, because the supplier leaves room and sells by the tank rather than by the pound. A 15 lb fill holds 323,220 BTU, which is 10.8 hours on a 30,000 BTU grill instead of 14.4. If you want the full 20 pounds, get the tank refilled at a station that charges by the gallon and check the scale ticket.

Weighing is the honest method. Every cylinder has a tare weight stamped on the collar, usually TW followed by a number around 17 or 18 pounds for a 20 lb tank. Put the tank on a bathroom scale, subtract the tare weight, and divide by 20. A reading of 27 pounds total means 9 or 10 pounds of propane, which is roughly half a tank and about seven hours of grilling.

Cold weather, vaporization and the draw limit

Propane has to boil before it burns. The liquid in the tank absorbs heat from the walls and turns into vapor, and the burner pulls that vapor out. Cold slows the boil, and a small tank has less surface area to absorb heat from, so there is a ceiling on how many BTU per hour any given tank can deliver.

For a 20 lb cylinder the ceiling sits near 100,000 BTU per hour in mild weather and drops to roughly half that around freezing. That is why a 20 lb tank cannot feed a 150,000 BTU tankless water heater even though the division says it holds enough fuel for nearly three hours. The tank can hold the heat, it just cannot release it fast enough. A tankless unit needs a 100 lb cylinder or a house tank, and the installation manual will say so. The NFPA 58 standard is the code that governs cylinder fill limits, relief valves and storage.

This is also why tanks frost on the outside. As vapor leaves, the liquid cools, and moisture in the air freezes on the steel. A heavy frost line on the bottom half of a tank during use means the draw is close to the limit. Warm the tank with a blanket of room temperature air, never with a torch or a heat lamp, and split the load across two tanks if the appliance is large.

Practical numbers for a cold night: a 20 lb tank on a 48,000 BTU patio heater will run, but the pressure drops as the tank empties and the flame gets smaller. The heater will not hold its rated output for the last hour. Plan on about 80 percent of the calculated run time when the temperature is below 40 degrees Fahrenheit.

Cost per hour and what to check before a refill

Cost per hour is the number that makes two appliances comparable. Divide the BTU per hour by 91,500 to get gallons per hour, then multiply by the price per gallon. At $3.50 a gallon a 30,000 BTU grill costs $1.15 an hour and a 48,000 BTU patio heater costs $1.84 an hour. A heat pump or a natural gas line usually beats both, but propane wins where there is no gas main.

Prices swing by season and by region, and the spread between a refill and an exchange is often 30 percent or more per gallon once you account for the short fill. Refills at a station with a scale are usually the cheapest option, and buying 100 pounds at a time lowers the per gallon price further on a house account.

Before a refill, check three things. The date stamp on the collar shows when the cylinder was made, and most suppliers will not refill a tank older than 12 years without a recertification. The valve should be an OPD, the overfill protection device required on all cylinders made since 1998. And the collar and foot ring should be free of deep rust, since a rusted foot ring is what fails first.

Store propane outside, upright, and away from the house. A cylinder that has been in a hot garage all afternoon holds more pressure than one in the shade, and the relief valve will vent to protect the tank. That venting is propane you paid for going into the air, and it is the main reason a tank seems to lose fuel when it sits.

The figures here are estimates for planning. Tank capacity, fill limits and appliance ratings vary, and NFPA 58 sets the rules for storage and filling. Check the label on your appliance and the stamp on your cylinder before you rely on a number.