About the Propane Tank Duration Calculator
This propane tank calculator tells you how long the propane you have will last. Pick your tank — from a 20 lb barbecue cylinder to a 1,000 gallon home tank — enter the gauge reading, the BTU rating of the appliance (or appliances) it feeds and how many hours a day they run, and it returns the usable gallons, total runtime in hours and the number of days until you hit your refill point.
It is useful for homeowners deciding when to book a delivery before a cold snap, RV owners and campers planning a trip, and anyone sizing a cylinder for a patio heater, generator or grill. Add your price per gallon to see the daily fuel cost and the value of the propane left in the tank.
Large tanks are filled to about 80% of their water capacity to leave room for expansion, and their gauges read the percentage of total capacity — so a full 500 gallon tank holds about 400 gallons. Cylinders are rated by the pounds of propane they hold when full; the calculator converts pounds to gallons at about 4.24 lb per gallon. Appliance BTU ratings are maximum input, so real-world runtime is often longer when burners or furnaces cycle.
With the default inputs, the days until refill is 57.2 days. Change any value above to recalculate instantly.
How to use the propane tank duration calculator
- 1Choose your tank or cylinder size (or enter a custom capacity in gallons).
- 2Enter the gauge reading — about 80% for a freshly filled tank, 100% for a full cylinder.
- 3Set the level at which you plan to refill, such as 20%.
- 4Enter the BTU/h rating of the appliances using the propane and how many hours a day they burn.
- 5Read the days until refill, burn rate and daily cost.
Formula and method
Usable gallons are the tank capacity multiplied by the gauge reading minus the refill reserve. For cylinders sold by weight, capacity is the rated pounds divided by 4.24 lb per gallon, so a full 20 lb cylinder holds about 4.7 gallons. Each gallon of propane contains about 91,452 Btu of energy (U.S. EIA).
Dividing the usable energy by the combined BTU/h rating of your appliances gives total burn time at full fire. Dividing by your daily hours of use gives the number of days. Because furnaces, heaters and burners cycle on and off or run below maximum, real runtime is often longer than this full-load estimate — enter realistic hours of actual burning rather than hours switched on.
- Usable gallons
- Capacity × (gauge % − refill %) ÷ 100
- 91,452
- Btu per gallon of propane (EIA)
- Load
- Total BTU/h input rating of appliances running together
- 4.24
- Pounds of propane per gallon
Worked examples
500 gallon tank heating a home
A 500 gallon tank at 80% with a 20% refill point leaves 300 usable gallons, or about 27.4 million Btu. An 80,000 BTU/h furnace burning 6 hours a day uses about 5.25 gallons a day, so the propane lasts roughly 57 days and costs about $13.65 a day at $2.60/gal.
20 lb cylinder on a gas grill
A full 20 lb cylinder holds about 4.72 gallons (20 ÷ 4.24), or roughly 431,000 Btu. A three-burner grill rated 36,000 BTU/h on full for one hour a day would empty it in about 12 hours of cooking — about 12 days.
100 lb cylinder for a patio heater
Three-quarters of a 100 lb cylinder is 75 lb, or about 17.7 gallons. A 40,000 BTU/h patio heater would run about 40 hours on that, which is roughly 10 evenings of 4 hours.
Frequently asked questions
How long does a 20 lb propane tank last?+
A full 20 lb cylinder holds about 4.7 gallons or roughly 430,000 Btu. On a 36,000 BTU/h grill at full flame that is about 12 hours; on a 10,000 BTU/h heater it is about 43 hours. Lower settings stretch it further.
How many gallons are in a full 500 gallon propane tank?+
About 400 gallons. Tanks are filled to roughly 80% of their water capacity to leave space for the liquid to expand as it warms, so a 500 gallon tank holds around 400 gallons and a 250 gallon tank around 200.
How much propane does a furnace use per hour?+
Divide the furnace input rating by 91,452 Btu per gallon. A 100,000 BTU/h furnace burns about 1.09 gallons per hour of run time, and a 60,000 BTU/h unit about 0.66 gallons per hour.
When should I refill my propane tank?+
Most propane suppliers recommend scheduling a refill when the gauge reads 20–30%, especially in winter when deliveries can be delayed. Running a tank empty can let air and moisture in and may require a leak check before relighting.
How do I read a propane tank gauge?+
The dial under the lid of a large tank shows the percentage of the tank’s total capacity that is filled with liquid propane. Multiply the reading by the tank size: 40% on a 500 gallon tank is about 200 gallons.