Find out how long your propane will last before the grill goes cold or the heater sputters out. Pick a tank from a 20 lb cylinder up to a 1,000-gallon home tank, enter the appliance's BTU rating, and the calculator shows hours of runtime, burn rate in gallons and pounds per hour, and the cost per hour at your local price. It can also tell you how full a cylinder is from its weight on a bathroom scale.
Sizing a heater for a room? The BTU calculator estimates the heating load. Planning for an outage, the battery runtime calculator compares battery backup, and the UPS runtime calculator shows how long a battery bank carries the same load before the generator has to take over. The math behind every number here, from gallons in a 20 lb tank to burner on-time, is worked through in how long a propane tank lasts.
Frequently asked questions
How long does a 20 lb propane tank last?
A full 20 lb cylinder holds about 4.7 gallons of propane, or roughly 430,000 BTU. Divide that by the appliance's BTU rating to get hours. A 36,000 BTU three-burner grill on high runs about 12 hours; a 40,000 BTU patio heater runs about 10–11 hours; a 12,000 BTU portable grill lasts around 36 hours. Most people cook at medium, not full flame, so real-world grilling often stretches a tank to 18–20 hours.
How do I tell how much propane is left in my tank?
For a grill-size cylinder, weigh it. Every DOT cylinder has a tare weight stamped on the collar, marked TW — that is its empty weight, usually 15–18 pounds for a 20 lb tank. Subtract the tare from the scale reading to get pounds of propane left; a 20 lb tank that weighs 27 pounds with a TW of 17 has 10 pounds left, so it is half full. Large home tanks have a float gauge that reads the percentage of total tank capacity instead.
Why are big propane tanks only filled to 80 %?
Liquid propane expands noticeably as it warms up — much more than water. Safety codes require large tanks to be filled only to about 80 % of their water capacity so there is vapor space for that expansion. That means a 500-gallon tank holds about 400 gallons of propane when full, and its gauge will read 80 %, not 100 %. Grill cylinders have an overfill protection device that does the same job automatically.
How much propane does a generator use?
Check the spec sheet first — manufacturers list propane use at half and full load. As a rule of thumb, a propane generator burns about 0.22 gallons per hour for every kilowatt of actual load (roughly 20,000 BTU per kWh, calibrated to Generac's published 22 kW figures at half load). A 7 kW portable at half load uses roughly 0.77 gallons an hour, and a 22 kW standby unit at half load about 2.4 gallons an hour; at full load the same rule runs about 30 % high, because big engines are more efficient near full output. Use the 'Generator by load' option here, or type the spec-sheet figure into 'Gallons per hour'.
Why does my furnace use less propane than this calculation?
The BTU rating is the input at full flame, but furnaces, water heaters, and fireplaces cycle on and off with the thermostat. A furnace might fire 30–50 % of the time on a cold day and much less in mild weather. Set the burner on-time percentage to reflect that. Cold also works the other way for small cylinders: below about 20 °F, a 20 lb tank may not vaporize propane fast enough to feed a big burner, so it runs weaker or freezes up before it is empty.