Find out how long a propane tank will run a grill, heater, RV furnace or home furnace, from the tank size, how full it is and the appliance’s BTU rating. A 20 lb cylinder on a grill and a 500 gallon tank on a house both work the same way.
Uses 91,500 BTU per gallon and 4.24 lb per gallon of propane. Cylinders are rated by propane weight, so a blank percent field means a full cylinder (100%). Large tanks are rated by water capacity and filled to about 80%, so for them the field takes your gauge reading and a blank field uses 80%. A blank burner setting counts as 100% and blank hours per day as 24 (non-stop). Real run time drops in cold weather and as appliances cycle, so treat the result as an estimate.
How Long Does a 20 lb Propane Tank Last on a Grill?
A full 20 lb tank runs a 30,000 BTU grill for about 14.4 hours at full power. The tank holds roughly 4.7 gallons, or 431,600 BTU, and 431,600 ÷ 30,000 = 14.4. Cook on medium (about 75%) and it stretches to around 19 hours. Smaller or bigger burners change it a lot:
| Appliance rating | 20 lb tank | 40 lb tank | 100 lb tank |
|---|---|---|---|
| 18,000 BTU/hr | 24.0 hours | 48.0 hours | 119.9 hours |
| 30,000 BTU/hr | 14.4 hours | 28.8 hours | 71.9 hours |
| 40,000 BTU/hr | 10.8 hours | 21.6 hours | 54.0 hours |
| 50,000 BTU/hr | 8.6 hours | 17.3 hours | 43.2 hours |
How Many BTU Are in a 20 lb Propane Tank, and How the Math Works
A 20 lb propane tank holds about 431,600 BTU, because each gallon of liquid propane carries about 91,500 BTU. Every appliance has a rated input in BTU per hour on its data plate or in the manual. Divide the energy in the tank by the rate the appliance burns it, and you have the run time. The tool first works out how many gallons are really in the tank, then applies your burner setting, because a grill on medium or a furnace that cycles on and off uses less than its full rating.
Formula: Gallons in a cylinder = Rated pounds ÷ 4.24 × Percent full ÷ 100. Gallons in a large tank = Tank size × Gauge reading ÷ 100. Energy (BTU) = Gallons × 91,500. Burn rate = BTU/hr rating × Burner setting ÷ 100. Run time (hours) = Energy ÷ Burn rate. Days = Run time ÷ Hours per day.
- Turn pounds into gallons: 40 ÷ 4.24 = 9.43 gallons.
- Turn gallons into energy: 9.43 × 91,500 = about 863,200 BTU.
- Divide by the burn rate: 863,200 ÷ 30,000 = 28.8 hours.
- Divide by hours used per day to get days.
That is how long a 40 lb propane tank will last at 30,000 BTU: just under 29 hours of burner time.
- Tank size: portable cylinders are sold by the pounds of propane they hold; stationary tanks by gallons of water capacity.
- Percent full / Gauge reading (optional): how much is left. Blank means a full cylinder, or 80% for a large tank, which is the usual maximum fill.
- Appliance rating: the maximum input in BTU per hour. Add the ratings together if several appliances share the tank.
- Burner setting (optional): the share of full power you actually use, or the share of time a thermostat keeps the burner lit. Blank counts as 100%.
- Hours per day (optional): how long the appliance runs each day. Blank assumes non-stop use, 24 hours a day.
Run Times for Grills, Heaters and Furnaces
| Tank and fill | Appliance | Setting | Hours per day | Propane | Run time | Days |
|---|---|---|---|---|---|---|
| 20 lb, full | Grill, 30,000 BTU/hr | 100% | 24 | 4.7 gal | 14.4 hours | 0.6 |
| 20 lb, full | Patio heater, 40,000 BTU/hr | 100% | 24 | 4.7 gal | 10.8 hours | 0.4 |
| 40 lb, full | RV furnace, 50,000 BTU/hr | 50% | 24 | 9.4 gal | 34.5 hours | 1.4 |
| 120 gal, 80% gauge | Furnace, 60,000 BTU/hr | 100% | 6 | 96.0 gal | 146.4 hours | 24.4 |
| 500 gal, 65% gauge | Furnace, 100,000 BTU/hr | 100% | 10 | 325.0 gal | 297.4 hours | 29.7 |
The furnace rows treat Hours per day as the total time the burner is lit, which is how a thermostat-controlled heater behaves.
How Many Days Will a 500 Gallon Propane Tank Last?
A 500 gallon tank filled to 80% runs a 100,000 BTU furnace for about 36.6 days if the burner is lit 10 hours a day. It holds 400 gallons, or 36.6 million BTU. Real heating use swings with the weather, so treat that as a starting point and check these things too.
- Weigh, do not guess. An empty 20 lb cylinder weighs about 17 lb, stamped as TW on the collar. Subtract that from the scale reading to find the pounds left.
- Read the gauge as a share of the whole tank. A large tank reading 80% is full. A 100 gallon tank holds about 80 gallons, not 100.
- Heating loads change with weather. How long a big tank lasts depends on outdoor temperature, insulation and the thermostat. Track burner hours for a cold week.
- Cold slows vaporization. A small cylinder in freezing weather may not deliver its full BTU rating, so heaters sputter well before the tank is empty.
- Order early. Refill around 20 to 30% on the gauge so a delivery delay does not leave you cold.
For the energy content and physical properties behind these numbers, see Wikipedia’s article on propane as a fuel, and follow your appliance manual and local codes for safe tank placement.
Common questions about propane run time
How long does a 20 lb propane tank last on a grill?
A full 20 lb tank holds about 431,600 BTU, so a 30,000 BTU grill on high runs about 14.4 hours. At a 75% burner setting it lasts about 19 hours.How many BTU are in a 20 lb propane tank?
A full 20 lb cylinder holds about 4.7 gallons of propane. At roughly 91,500 BTU per gallon, that is about 431,600 BTU. Divide that by your appliance rating per hour to get the run time.How long will a 100 lb propane tank last at 30,000 BTU?
A full 100 lb cylinder holds about 23.6 gallons, or 2.16 million BTU, so a 30,000 BTU appliance at full power runs roughly 72 hours. A generator’s real fuel use depends on its load, so check the manual.How many days will a 500 gallon propane tank last?
Filled to the usual 80% it holds 400 gallons, or 36.6 million BTU. A 100,000 BTU furnace lit 10 hours a day would use that in about 36.6 days, though cold snaps shorten it.How do I find my appliance BTU rating?
Look for the data plate or rating label, usually inside a door, on the back panel or near the gas valve. If several appliances share one tank, add their ratings together for a worst-case run time.Why does my tank run out sooner than expected?
Common causes are a burner set higher than you think, a furnace that runs longer in cold snaps, a leak, or a gauge that reads high. Small cylinders also struggle to vaporize gas in freezing weather.Related Calculators
Compare what each heat source costs with the electricity cost calculator for appliances, or price a refill run with the fuel cost calculator for trips. This tool pairs well with the CO2 footprint calculator if you want to see the emissions side of your heating or grilling.