How long does a 100 lb propane cylinder last?
At full fire, a filled 100 lb cylinder runs a 26,000 Btu/h fireplace for about 83 hours and a 30,000 Btu/h heater for about 72 hours. The rule is one line: the cylinder's gallons times 91,452 Btu per gallon, divided by what the appliance burns per hour. Enter yours below.
Hours of running time by cylinder and appliance
Each cell is one filled cylinder burning at the full rating of the appliance in the column heading. Real burners cycle or turn down, which stretches the numbers.
| Cylinder | Gallons | 10,000 Btu/h | 26,000 Btu/h | 40,000 Btu/h | 60,000 Btu/h | 80,000 Btu/h | 100,000 Btu/h |
|---|---|---|---|---|---|---|---|
| 20 lb | 4.7 | 43.1 | 16.6 | 10.8 | 7.2 | 5.4 | 4.3 |
| 40 lb | 9.4 | 86.3 | 33.2 | 21.6 | 14.4 | 10.8 | 8.6 |
| 100 lb | 23.6 | 215.7 | 83.0 | 53.9 | 35.9 | 27.0 | 21.6 |
| 420 lb | 99.1 | 905.9 | 348.4 | 226.5 | 151.0 | 113.2 | 90.6 |
Divide the hours by the time the appliance runs each day to get days. A 100 lb cylinder on the 26,000 Btu/h column is 83.0 hours, which is 21 days at 4 hours a day. Home heating needs far more: see how much propane a furnace uses, or the propane tank percentage chart for a tank with a gauge.
Why some pages give a lower number
A cylinder's pound rating is the propane it holds when filled. Take Powerblanket's 100 lb runtime page: it says "A 100-pound propane tank has a total capacity of approximately 23.6 gallons of propane", then "distributors normally fill the tanks to only 80% of their capacity, resulting in about 18.9 gallons of usable propane". The first figure is 100 lb divided by 4.24 lb per gallon, which I read as the filled cylinder already, so the 80% looks applied a second time. The page then gets 1,729,350 Btu and 57.6 hours at 30,000 Btu/h. Without the second 80% the same sums give 71.9 hours. (That page also gives a practical 26 to 28 hours for losses, which I do not model. Its own table says 32 to 34 hours.)
Hunker uses the full 100 lb: "One gallon of propane provides 91,500 BTU of heat energy and weighs 4.25 pounds", and for a 26,000 Btu/h fireplace "your 100-pound bottle will fuel your propane fireplace for about 84 hours". Their 84 hours is about 1% above my 83.0, which is within the rounding of their constants.
Canadian suppliers show the same thing in litres. Superior Propane writes "A single 420 lb. propane cylinder at maximum capacity (80%) can hold up to 372 litres of propane." At 4.24 lb per gallon, 420 lb is 375 L, within about 1% of their 372 L. My reading is that the 80% in their sentence is the fill limit already built into the 420 lb rating, so it is not taken off again. Check with your supplier if you need the exact figure for a cylinder.
The math and my assumptions
gallons = pounds ÷ 4.24
hours = gallons × 91,452 ÷ appliance Btu/h
days = hours ÷ hours per day
- Energy per gallon: 91,452 Btu (EIA). Powerblanket and Hunker use 91,500, and Suburban Propane says "around 91,500 BTUs".
- Weight per gallon: 4.24 lb (Engineering Toolbox LPG table). Hunker uses 4.25, and the real value shifts a little with temperature.
- The cylinder is full. A partly used one lasts in proportion: half full, half the hours.
- Cold weather, a regulator or a vaporization limit can cut what a cylinder delivers. This page does not model them. Follow the cylinder and appliance manuals for safe use.
Try a few ready-made cases: a 20 lb grill cylinder at 30,000 Btu/h, a 100 lb cylinder on a 50,000 Btu/h fire pit or a 420 lb cylinder on an 80,000 Btu/h furnace.