A cylinder is a battery with a fixed charge. Runtime is that charge divided by how fast the burner spends it, and almost everyone underestimates how much the flame knob changes the answer.
A full 20 pound cylinder holds about 4.7 gallons of liquid propane, which is roughly 431,000 BTU of stored energy. On a 50,000 BTU fire pit burner at full flame that works out to 8.6 hours. Drop to half flame and the same tank gives you about 17 hours, or six or seven evenings outside.
Exchange cages price by the tank, not the gallon. Divide the swap price by 3.5 gallons to get a comparable number, and by 4.7 gallons if you refill by weight at a propane dealer.
Every propane runtime question reduces to one division problem. Stored energy divided by hourly draw equals hours. The only trick is getting both numbers right, and the second one is where most published estimates fall apart.
Start with the fuel. The U.S. Energy Information Administration puts propane at roughly 91,452 BTU per gallon, and liquid propane weighs about 4.24 pounds per gallon at 60 degrees Fahrenheit. That gives you two conversions worth memorizing: about 21,570 BTU per pound of propane, and about 4.7 gallons in a full 20 pound cylinder. Multiply those out and a standard grill tank carries around 431,000 BTU.
Now the draw. A burner rated at 50,000 BTU means 50,000 BTU per hour at full output, measured with the valve wide open. Fire pit burners are not efficiency rated the way a furnace is, because every bit of that heat goes straight into the air by design. What the rating tells you is fuel consumption, and fuel consumption is what runtime depends on.
The table below assumes a full cylinder and a burner running at its rated output. Find your burner rating on the left, then read across to your cylinder. If you do not know your rating, our fire pit BTU guide covers how output is matched to patio size and where manufacturers tend to overstate it.
| Burner rating | 15 lb exchange | 20 lb refill | 30 lb | 40 lb | 100 lb |
|---|---|---|---|---|---|
| 30,000 BTU | 10.8 h | 14.4 h | 21.6 h | 28.8 h | 71.9 h |
| 40,000 BTU | 8.1 h | 10.8 h | 16.2 h | 21.6 h | 53.9 h |
| 50,000 BTU | 6.5 h | 8.6 h | 12.9 h | 17.3 h | 43.1 h |
| 60,000 BTU | 5.4 h | 7.2 h | 10.8 h | 14.4 h | 36.0 h |
| 75,000 BTU | 4.3 h | 5.8 h | 8.6 h | 11.5 h | 28.8 h |
| 90,000 BTU | 3.6 h | 4.8 h | 7.2 h | 9.6 h | 24.0 h |
| 110,000 BTU | 2.9 h | 3.9 h | 5.9 h | 7.8 h | 19.6 h |
Almost nobody runs a fire pit wide open all evening. The flame goes up for the first ten minutes, then comes down once the seating area warms. That single habit changes the numbers more than tank size does.
| Flame position | Fuel draw | Runtime | Evenings at 3 hours | Cost per hour at $3.50 per gallon |
|---|---|---|---|---|
| Full | 50,000 BTU per hour | 8.6 h | 2.9 | $1.91 |
| Three quarters | 37,500 BTU per hour | 11.5 h | 3.8 | $1.44 |
| Half | 25,000 BTU per hour | 17.3 h | 5.8 | $0.96 |
| One third | 16,500 BTU per hour | 26.1 h | 8.7 | $0.63 |
| Low | 12,500 BTU per hour | 34.5 h | 11.5 | $0.48 |
Runtime against flame position is not a straight line. It is a hyperbola, which means the payoff for backing off the knob accelerates the lower you go. Going from full to three quarters buys you three extra hours. Going from half to one third buys you nine. The chart below updates with whatever you set in the calculator above.
There is a practical floor. Turn a fire pit burner too low and the flame gets lazy, the media in the bowl stops radiating, and you lose the heat you came outside for. Most propane pits sit in a comfortable band between one third and two thirds, and that band is where the fuel math actually lives.
A 20 pound cylinder is not full of 20 pounds of anything when you pick it up. It holds roughly 47 pounds of water if you filled it to the brim, and federal rules cap how much propane goes in. Filling limits for liquefied petroleum gas in DOT specification cylinders are set in 49 CFR 173.304a, which expresses the limit as a filling density, meaning the weight of gas as a percentage of the weight of water the container would hold. For propane that works out to roughly 42 percent, which is where the 20 pound figure comes from and why the liquid stops around four fifths of the way up.
That leftover space is not wasted. Liquid propane expands as it warms, far more than water does, and the vapor space is what absorbs that expansion instead of the pressure relief valve. It is also the space where liquid boils into the gas that feeds your burner.
Swap programs almost always fill to 15 pounds rather than 20, on a cylinder rated for 20. You are buying three quarters of a tank at close to a full tank price, and your fire pit runs a quarter shorter than every chart on the internet says it should. A dealer who refills by weight is cheaper per BTU and gives you the runtime the burner was designed around.
Here is the part that surprises people who buy a large fire pit table and hook it to one grill cylinder. Your burner does not draw liquid. It draws vapor, and vapor only exists because liquid propane is boiling against the inside wall of the tank. Boiling takes heat, and the only heat available is whatever the surrounding air pushes through the steel.
That gives every cylinder a maximum continuous output, independent of how much fuel is inside. Vaporization tables published for propane equipment put a full 100 pound cylinder near 300,000 BTU per hour at 70 degrees Fahrenheit. Scale that down to the wetted surface of a 20 pound cylinder and you land in the neighborhood of 60,000 to 70,000 BTU per hour when full and warm, dropping steadily as the liquid level falls and less steel is in contact with it.
So a 90,000 BTU fire pit table on a single 20 pound cylinder cannot hold 90,000 BTU. What happens instead is familiar to anyone who has owned one: the flame looks great for fifteen minutes, frost creeps up the outside of the tank, the flame drops, and it never comes back until the tank warms up. Nothing is broken. The tank simply cannot boil fuel fast enough.
If your pit is rated above about 65,000 BTU and you use it often, a permanent gas line is the better answer to this problem. Our guide to running a gas line to a fire pit covers trench depth, pipe sizing, and where an inspection is required, and propane versus natural gas compares what each fuel costs to run.
Propane is stored as a liquid because pressure keeps it there, and that pressure comes entirely from temperature. Propane boils at about 44 degrees below zero Fahrenheit at atmospheric pressure, which is why it works outdoors in winter at all when butane camping fuel quits near freezing. But the pressure available to push gas through your regulator falls sharply as the air cools.
Two things follow from that curve. First, the stored energy in the tank does not change with temperature, so your total runtime figure stays the same. Second, the rate at which you can pull it out drops, which means on a cold night a large burner behaves like a smaller one whether you asked it to or not. In practical terms, a fire pit that runs beautifully in October may need the valve backed off in January to keep a steady flame.
Storing a cylinder in a heated garage to fight this is the wrong trade. Federal workplace rules on liquefied petroleum gas in 29 CFR 1910.110 restrict where cylinders can be stored indoors, and consumer guidance from fire agencies is consistent: cylinders belong outdoors, upright, away from ignition sources and building openings. Keep the tank out of the shade instead, and let the sun do the work during the day.
Cost per hour is a two step conversion. Divide burner BTU per hour by 91,452 to get gallons per hour, then multiply by what you pay per gallon. A 50,000 BTU burner burns about 0.55 gallons per hour at full flame, so the arithmetic is simple once you know your local price. The Energy Information Administration publishes weekly residential propane prices by state through the heating season, which is a better reference point than whatever the exchange cage outside the grocery store charges.
| Burner rating | Gallons per hour, full flame | Cost per hour, full flame | Cost per hour, half flame | Cost per 3 hour evening, half flame |
|---|---|---|---|---|
| 30,000 BTU | 0.33 | $1.15 | $0.57 | $1.72 |
| 50,000 BTU | 0.55 | $1.91 | $0.96 | $2.87 |
| 65,000 BTU | 0.71 | $2.49 | $1.24 | $3.73 |
| 90,000 BTU | 0.98 | $3.44 | $1.72 | $5.17 |
Set against firewood, propane usually loses on raw fuel cost and wins on everything else, since there is no hauling, no ash, and no waiting for coals. Set against a plumbed natural gas line, propane loses badly on cost per BTU and wins on portability. If the pit never moves and you use it most weekends, the line pays for itself. If it moves around the yard or comes off the deck in winter, the cylinder is the right call. Our comparison of propane fire pit tables lists rated output alongside build quality, which is the number that drives all of this.
Inline pressure gauges are close to useless for this. They read vapor pressure, and vapor pressure stays roughly constant while liquid remains, then falls off a cliff at the end. What that gauge is really telling you is the temperature of your tank. Weight is the only honest measurement, and it takes about a minute.
Uses the burner rating and flame position you set in the calculator above, so the hours figure matches your own pit.
The hot water trick works too when you have no scale. Pour a cup of warm water down the side of the tank and run your hand along the steel. The wet metal is cool where liquid absorbs the heat and warm where only vapor sits, and the boundary is your fuel level. It tells you where the line is, not how many hours are behind it.
Runtime planning is not much use if the pit is in the wrong place. The U.S. Fire Administration advises keeping outdoor fireplaces and fire pits at least ten feet from the home or anything that can burn, and keeping children supervised whenever the pit is lit. Local fire code often goes further, and our clearance guide and the piece on how far a fire pit should sit from a house break down the common requirements.
Lighting sequence matters too, especially on a pit that has sat unused. Open the lid or cover, purge the line, and light on the first attempt rather than letting gas pool in the bowl. The full sequence is in how to light a gas or propane fire pit.
About 8.6 hours on a 50,000 BTU burner at full flame, and roughly 17 hours at half flame. The stored energy is fixed at around 431,000 BTU, so everything depends on how fast the burner spends it.
Around 431,000. A 20 pound fill is close to 4.7 gallons, and federal energy figures put propane near 91,452 BTU per gallon, which is about 21,570 BTU per pound.
Most swap programs fill to 15 pounds, not 20. That is a quarter less fuel in a cylinder rated for more, so a burner that should give you 8.6 hours gives you about 6.5.
Not at full output for long. A cylinder can only boil so much liquid per hour, roughly 60,000 to 70,000 BTU worth when full and warm, and less as it empties. The flame will fall back on its own and the tank will frost. Use a 40 pound cylinder, two cylinders on a tee, or a plumbed line.
Yes, and close to proportionally. Half output means roughly half the gallons per hour. It is the only adjustment that reliably doubles an evening.
Burner BTU divided by 91,452 gives gallons per hour. A 50,000 BTU burner is about 0.55 gallons per hour, so near three dollars per gallon that is roughly a dollar and 64 cents per hour wide open, and half that dialed back.
Tank pressure tracks temperature. Near 70 degrees a cylinder sits around 110 psi, and near zero degrees it falls to roughly 24 psi. The fuel is still in there, but it cannot boil into vapor fast enough to feed a large burner.
Not for measuring fuel. It reads pressure, which barely moves until the liquid is nearly gone, and it swings with air temperature. Weigh the cylinder instead and subtract the tare weight stamped on the collar.
They light and run, but thinner air means less oxygen per unit volume, so burners tend to run slightly richer and produce a lazier, taller flame. Some manufacturers list high altitude orifice kits for elevations above roughly 4,500 feet.
FirePitSpot is reader supported and may earn a commission from qualifying purchases linked from this site. Runtime figures are calculated from published fuel energy values and burner ratings. Real results vary with altitude, air temperature, wind, regulator condition, and how accurately a burner is rated.