How long does a 20 lb propane tank last?
A full 20 lb tank holds about 431,377 Btu. That is roughly 18 hours on a gas grill burning 24,000 Btu/h and 9 hours on a 48,000 Btu/h patio heater, both at full output. At half output the hours double.
Hours by appliance
Ratings are the Btu per hour the manufacturer's page lists. A full tank, burning at that rating or at half of it.
| Appliance | Btu/h | Full output | Half output |
|---|---|---|---|
| Mr. Heater Buddy, high | 9,000 | 47.9 h | 95.9 h |
| Weber Spirit E-310 grill, main burners | 24,000 | 18.0 h | 35.9 h |
| Mr. Heater 48,000 Btu patio heater | 48,000 | 9.0 h | 18.0 h |
| Mr. Heater Buddy, low | 4,000 | 107.8 h | 215.7 h |
- Weber lists the Spirit E-310 (liquid propane) at 24,000 Btu per hour for the main burners. Other listings may show a different total, so check your grill's own plate.
- Mr. Heater's 48,000 Btu patio heater page says "Operates up to 10 hours". The sum gives 9.0 hours at the full rating, so the claim fits a slightly lower burn.
- The Buddy heater is rated "4,000 or 9,000 BTU per hour" and "Connects directly to a 20 lb. cylinder with optional hose and filter". Its manual and the maker's warnings decide where it can be used.
The arithmetic
20 lb ÷ 4.24 lb per gal = 4.720 gal
4.720 gal × 91,452 Btu = 431,377 Btu
hours = 431,377 ÷ appliance Btu/h
The 20 lb on the cylinder is the propane it holds when filled, so no further 80% comes off. Powerblanket says a 20 lb tank "holds 4.6 gallons of propane" and that a gallon of propane "produces about 92,000 BTUs". Both are close to the figures here. The energy per gallon is the EIA number and the weight per gallon is from the Engineering Toolbox LPG table.
A worked example
A fire pit with a 30,000 Btu/h burner, run 2 hours an evening. 431,377 ÷ 30,000 is 14.4 hours, which is 7 evenings at full flame. On the half setting the same tank is 28.8 hours and 14 evenings.
Where the quoted hours differ
Powerblanket's 20 lb page says "18 to 20 hours on a medium grill" and "around 10 hours" for a large grill. The first matches a 24,000 Btu/h grill at full output (18.0 h) or a bit less. The second is a grill burning about 43,000 Btu/h. Grills differ, so the range comes from their ratings and knob settings.
The same page notes that "a full tank will read as less than full in cold weather" and that "cold tanks also run the risk of not having enough pressure to emit propane when needed." This page does not model cold. A partly used tank lasts in proportion: half full, half the hours.
Related pages
To enter your own appliance rating and hours a day, or to compare with larger cylinders, use the 100 lb propane cylinder page (a 100 lb cylinder runs a 26,000 Btu/h fireplace about 83 hours). A tank with a gauge is on the propane tank percentage chart.