The short answer: a full 20 lb tank feeding a 50,000 BTU burner wide open lasts roughly 9 hours. Turn the flame down to half and that stretches to somewhere near 17 to 18 hours, because a propane fire pit burns fuel in rough proportion to how far the valve is open, not on some fixed schedule.
The math behind the number
A standard 20 lb propane tank holds close to 4.7 gallons of liquid propane. The Department of Energy's Alternative Fuels Data Center lists propane's energy content at roughly 84,250 to 91,500 BTU per gallon depending on whether the lower or higher heating value is used, which the U.S. Energy Information Administration tracks alongside every other common fuel. Multiply that out and a filled 20 lb tank is carrying somewhere around 400,000 to 430,000 BTU of stored energy, all of it waiting to be released through the burner.
A fire pit burner rated at 50,000 BTU running wide open spends that stored energy at 50,000 BTU per hour, which divides out to a little under 9 hours before the tank runs dry. Dial the flame down to half and the burner is only spending roughly 25,000 BTU per hour, which nearly doubles the runtime for the exact same tank. That relationship, burn rate times hours equals stored energy, is the entire calculation. Everything else in this guide is really just explaining why the real number sometimes drifts from that clean math.
Why a "20 lb" tank does not hold 20 lb of steel
It trips a lot of people up that a 20 lb propane cylinder is actually a container with a water capacity closer to 47 lb. The name refers to the weight of propane it is designed to hold, not the tank itself. NFPA 58, the Liquefied Petroleum Gas Code, requires every propane container to stop filling at 80 percent of its total water capacity, leaving a vapor space so the liquid has room to expand as it warms up. Run that math on a 47 lb water capacity cylinder and 80 percent works out to almost exactly 20 lb of propane, or the 4.7 gallons used throughout the calculator above. Filling stations and the Consumer Product Safety Commission both point to this same rule as one of the reasons newer tanks use overfill prevention valves rather than the older five prong style.
See it happen: an animated burn simulation
The gauge panel above includes a working simulation, not just a static number. Set a tank size, burner rating, and flame setting, then press "Watch it drain" to see the liquid level fall in compressed time while a countdown clock tracks the runtime in real hours and minutes. It is the same math as the reference table below, just animated so the relationship between flame setting and how fast the tank empties is easier to feel rather than just read.
Runtime by tank size and burner rating
| Tank size | Burner rating | Full flame runtime | Half flame runtime |
|---|---|---|---|
| 20 lb | 30,000 BTU | About 14.3 hrs | About 28.6 hrs |
| 20 lb | 50,000 BTU | About 8.6 hrs | About 17.2 hrs |
| 20 lb | 80,000 BTU | About 5.4 hrs | About 10.8 hrs |
| 30 lb | 50,000 BTU | About 12.9 hrs | About 25.8 hrs |
| 40 lb | 50,000 BTU | About 17.2 hrs | About 34.4 hrs |
| 100 lb | 50,000 BTU | About 43.0 hrs | About 86.0 hrs |
Why cold weather changes the real answer
The stored energy calculation assumes the tank can deliver gas as fast as the burner asks for it, and most of the time it can. Propane sits inside the tank as a liquid and has to vaporize into gas before it can reach the burner, and that conversion pulls heat from the surrounding air through the tank wall. A technical bulletin on tank capacity from regulator manufacturer Emerson/Fisher lays out how both a colder night and a tank running low at the same time shrink the wetted surface area inside the tank, which is exactly the area doing that heat transfer. On a mild evening with a fresh tank this is a non issue. On a cold night with a nearly empty tank and a large burner, the flame can weaken or sputter even though there is technically still propane inside, simply because the tank cannot boil off gas fast enough to keep up with demand.
Practically, this matters most for large fire pit tables in the 80,000 to 100,000 BTU range running on a tank that is already down below a quarter full in cold weather. If a flame that was steady all summer starts weakening on the first cold nights of the season, a low tank fighting the temperature is a more likely explanation than a fuel leak or a bad regulator.
Ways to actually stretch a tank
- Run a lower, steadier flame. Since burn rate scales with flame setting, a fire pit kept at medium rather than wide open buys nearly double the hours for the same tank, with very little visible difference in flame height once the sun goes down.
- Match the burner to the space, not the ceiling. An 80,000 BTU burner sized for a large open patio will drain a tank far faster than a 50,000 BTU unit sized to the actual seating area. The BTU guide walks through sizing a burner to square footage rather than defaulting to the biggest option.
- Start every gathering with a full tank. A tank sitting at a quarter full loses runtime twice over: less fuel in reserve, and a smaller wetted surface area to vaporize it from on a cool night.
- Keep the tank upright and away from direct heat. The U.S. Fire Administration recommends keeping fuel sources a safe distance from the fire itself and never storing or using a cylinder on its side, both of which also help the tank vaporize propane consistently.
- Check the connection before every season's first light. A slow leak at the hose or regulator wastes fuel without changing the flame appearance, which is part of why the Consumer Product Safety Commission recommends a soapy water leak check at the start of the season.
Frequently asked questions
How long does a 20 lb propane tank last on a fire pit?
At full output on a 50,000 BTU burner, a standard 20 lb tank runs for roughly 9 hours. Turning the flame down to medium can stretch that to somewhere around 15 to 18 hours, since fuel consumption scales closely with how far the valve is open.
How many BTU is a 20 lb propane tank?
A 20 lb tank holds roughly 4.7 gallons of propane. At the Department of Energy's figures of roughly 84,250 to 91,500 BTU per gallon, that works out to somewhere around 400,000 to 430,000 total BTU of stored energy, which is the number divided by burner output and flame setting to estimate runtime.
Does running the flame lower actually save propane?
Yes, close to proportionally. A burner running at half its rated BTU output burns through roughly half as much fuel per hour, so dialing back the flame is one of the most direct ways to stretch a tank.
Why is a 20 lb propane tank not actually 20 lb of steel capacity?
The 20 lb figure is the weight of propane the tank holds, not the tank itself. NFPA 58 limits any propane container to 80 percent of its water capacity for expansion room, and 80 percent of a standard cylinder's roughly 47 lb water capacity works out to close to 20 lb of propane.
Does cold weather make a propane tank run out faster?
Cold weather does not use up fuel any faster, but it can slow down how quickly liquid propane converts to usable gas inside the tank. On a cold night with a large burner and a tank running low, the pressure can drop enough that the flame weakens even though there is still liquid propane left inside.
More on FirePitSpot
Sources
- U.S. Department of Energy, Alternative Fuels Data Center: Fuel Properties Comparison
- U.S. Energy Information Administration: Units and calculators explained
- National Fire Protection Association: NFPA 58, Liquefied Petroleum Gas Code
- U.S. Consumer Product Safety Commission: Grill and propane cylinder safety checks
- U.S. Fire Administration: Outdoor fire safety
- Emerson/Fisher Regulators: How cold weather affects tank capacity