How this calculator works
Propane is sold by weight (cylinders) or by the gallon (bulk tanks), but appliances are rated in BTU per hour. One gallon of propane contains about 91,500 BTU, so the tank's energy divided by the appliance's use gives the hours.
Worked example: a 20 lb cylinder holds about 4.72 gallons, which is 431,880 BTU. A 3-burner grill rated at 36,000 BTU/hr burning flat out empties it in 12 hours. At 70% average power, it lasts about 17 hours. A bigger tank scales in proportion: a 100 lb cylinder lasts about five times longer.
Runtime at full power, by appliance and tank
Hours at the appliance's full BTU rating. Real use is usually below full power, so tanks last longer.
| Appliance | BTU/hr | 20 lb | 100 lb | 250 gal |
|---|---|---|---|---|
| Gas grill, 2-burner | 24,000 | 18.0 h | 90.0 h | 763 h |
| Gas grill, 3-burner | 36,000 | 12.0 h | 60.0 h | 508 h |
| Patio heater | 40,000 | 10.8 h | 54.0 h | 458 h |
| RV furnace | 30,000 | 14.4 h | 72.0 h | 610 h |
| Tank water heater | 40,000 | 10.8 h | 54.0 h | 458 h |
| Fire pit | 50,000 | 8.6 h | 43.2 h | 366 h |
| Gas range (everything on, max) | 65,000 | 6.6 h | 33.2 h | 282 h |
| Whole-house furnace | 80,000 | 5.4 h | 27.0 h | 229 h |
| Tankless water heater | 150,000 | 2.9 h | 14.4 h | 122 h |
Tips before you buy
- Weigh a cylinder to check how much is left. The tare weight (TW) is stamped on the collar: subtract it from the total, and the rest is propane (a full 20 lb tank weighs about 37 lb).
- In cold weather, propane vaporizes slowly and large appliances can starve on a small tank. Use a bigger tank, or keep it out of the cold, for heaters and generators.
- Never fill a tank above 80%: the space is needed for the liquid to expand. Never store or use propane indoors, in a garage or in a car.
- Check hoses and connections with soapy water for bubbles, before the season starts. If you smell gas, shut it off and get away from it.
- A regulator and gauge or a smart tank monitor takes the guesswork out of when to refill.
Quick answers by size
Tools & materials for this project
As an Amazon Associate we earn from qualifying purchases. These links do not change what you pay.
Frequently asked questions
How long does a 20 lb propane tank last on a grill?
About 12 hours of cooking at full power on a 3-burner grill (36,000 BTU/hr), and often 15 to 20 hours of real-world cooking, since grills rarely run flat out.
How many gallons are in a 20 lb propane tank?
About 4.7 gallons. Propane weighs about 4.24 lb per gallon.
How long does a 100 lb propane tank last?
About 23.6 gallons of propane, which is about 60 hours on a 36,000 BTU grill at full power, or about 27 hours on an 80,000 BTU furnace burning continuously.
How many BTUs are in a gallon of propane?
About 91,500 BTU. One pound holds about 21,600 BTU.
Add this calculator to your website
Running a blog, a contractor site or a class page? You can put this calculator on it for free. Copy the code below, and please leave the link underneath the calculator in place. The small script just resizes the frame to fit the results; if your site strips scripts, the calculator still works inside a scrolling frame.
Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.