The propane burn-time formula
Hours = gallons x 91,502 / BTU per hour. Gallons already include the fill percent. The burner setting scales the BTU per hour, so 50 percent uses half the rated BTU.
Worked example: 4.7 gallons x 91,502 / 36,000 = 11.9 hours. The same tank at a 50 percent setting lasts 23.9 hours. A 500 gallon bulk tank filled to the usual 80 percent holds 400 gallons. At 40,000 BTU per hour that is hundreds of hours, which is why the fill percent matters more than rounding the BTU constant to 91,500.
Which tank size to pick
The pound presets are the common portable cylinders: 20 lb for a grill, 30 lb and 40 lb for larger grills and some patio heaters, and 100 lb for a small fixed tank. Pound sizes are converted here at about 4.24 lb of propane per gallon (20 / 4.24 is about 4.7). If your cylinder is labeled in gallons already, the hour math only needs those gallons, the fill, and the burner.
Safety
Use propane appliances outdoors or where the manufacturer says they are allowed. A grill, patio heater, or fire pit indoors can produce deadly carbon monoxide. If you smell gas, shut the valve, get clear, and have a qualified technician check the connection. Do not estimate a leak with this calculator.
Frequently asked questions
How long does a 20 lb propane tank last?
About 11.9 hours on a 36,000 BTU per hour burner at full rate. The tank holds about 4.7 gallons, and each gallon is taken as 91,502 BTU. At a 50 percent burner setting the same tank lasts about 23.9 hours.
How many BTU are in a gallon of propane?
This calculator uses 91,502 BTU per gallon of propane. Hours equal gallons times 91,502, divided by the appliance BTU per hour, adjusted for fill level and burner setting.
Why are large propane tanks only filled to 80 percent?
Liquid propane expands as it warms. Bulk tanks such as a 500 gallon tank are filled to about 80 percent so that expansion has room. The 500 gallon preset on this page is 400 gallons of propane.
Does a lower burner setting make the tank last longer?
Yes. Burn time is inversely proportional to the BTU per hour you actually use. Half the flame setting uses half the fuel per hour, so the tank lasts about twice as long.