How to use the propane tank runtime calculator
- Pick a tank size preset or enter the pounds of propane in a full tank. A common grill tank is labeled 20 lb.
- Enter how full the tank is now, the appliance rating in BTU per hour and how much of that rating it uses on average.
- Optionally enter the hours of use per day to get a number of days.
- Open the advanced fields if you want to change the pounds per gallon, the BTU per gallon or a reserve you want to leave in the tank.
Formula
Usable pounds = tank pounds x (fill % - reserve %)
Gallons = usable pounds / pounds per gallon
Energy = gallons x BTU per gallon
Run time = energy / (appliance BTU per hour x average use %)
The defaults of 4.2 pounds per gallon and 91,500 BTU per gallon are approximate example values, and you can change both.
Worked example
A full 20 lb tank holds 20 / 4.2 = 4.76 gallons, which is about 435,714 BTU at 91,500 BTU per gallon. A grill burning 30,000 BTU per hour at full power empties it in about 14.5 hours.
Used 2 hours a day, that is roughly 7.3 days. A tank that is half full lasts half as long, and a burner that runs at half power lasts twice as long, so a half full tank at half power also gives about 14.5 hours.
What a tank label tells you
A 20 lb tank is sold with about 20 pounds of propane when full. The empty tank has its own weight, shown as the tare weight on the collar, so a scale reading minus the tare weight is the propane left. A gauge or the weighing method gives a better fill percent than guessing.
Burn rate is the big unknown
Appliance ratings are the maximum input, and most appliances run below it. A grill on low, a heater that cycles on a thermostat and a generator at partial load use less. Estimate the average percent of the rating, and note that cold weather can reduce how fast liquid propane vaporizes, which limits high draw from a small tank.
Related tools
To size heating needs use the BTU calculator, and to compare energy costs with electricity use the electricity cost calculator. For other containers see the tank volume calculator.
Assumptions and limits
- Pounds per gallon and BTU per gallon are approximate example values that vary with temperature and fuel mix, so they are editable.
- The appliance runs at a steady average percent of its rating.
- Pilot lights, efficiency losses and the last bit of fuel that is hard to use are not modeled. Use the reserve field to leave some behind.
- Always follow the safety instructions for your tank and appliance, and do not overfill a tank.
Frequently asked questions
How long does a 20 lb propane tank last?
On a 30,000 BTU per hour grill at full power, about 14.5 hours. At lower settings it lasts proportionally longer.
How many gallons are in a 20 lb propane tank?
About 4.76 gallons, using 4.2 pounds per gallon. The ratio changes slightly with temperature.
How do I know how much propane is left?
Weigh the tank and subtract the tare weight stamped on it, or use a gauge. Shaking or feeling for cold are rough guides only.
Why does my tank run out sooner than the estimate?
The appliance may run at a higher setting than you think, the weather may be cold, or the tank may not have been full to start with.