What the animation shows
The animation and the step-by-step video above use your numbers. Here is the same walkthrough written out for the default example.
What you enter
- Tank size: 20 lb cylinder (grill tank). Cylinders are sold by pounds of propane when full. Large tanks are sized by total (water) capacity and filled to about 80%.
- Appliances: 36000 × 1; 40000 × 2. Use the input rating from the appliance label or manual (BTU per hour at full flame). Hours are time the burner is actually lit: a furnace that cycles might burn 6–10 hours on a cold day.
What happens, step by step
- The 20 lb cylinder holds 4.7 gal, about 431,604 BTU.
- With every burner lit they draw 76,000 BTU/h: 5.7 hours of run time.
- At your hours per day they burn 1.27 gal/day.
- Each flame lights only for its share of every day.
- Empty after 3.7 days; keep a spare cylinder.
The answer: days per tank at your daily use is 3.7 days.
How it’s calculated
- Cylinder gallons = rated pounds ÷ 4.24 lb per gallon (20 lb ≈ 4.7 gal)
- Large tank gallons = tank capacity × gauge reading (full fill = 80%)
- Energy in the tank = gallons × 91,500 BTU per gallon
- Run time (hours) = energy ÷ total BTU/h of the appliances running together
- Days per tank = energy ÷ Σ(appliance BTU/h × burner hours per day)
Worked example
A 20 lb cylinder feeding a 36,000 BTU/h grill for 1 hour a day and a 40,000 BTU/h patio heater for 2 hours a day.
Inputs
- Tank size
- 20 lb cylinder (grill tank)
- Large-tank gauge reading
- 80 %
- Appliances
- 36000 × 1; 40000 × 2
- Propane price per gallon
- 0 $/gal
Results
- Days per tank at your daily use
- 3.7 days
- Run time with everything on at once
- 5.7 hours
- Propane in the tank
- 4.7 gal
- Propane weight
- 20 lb
- Energy in the tank
- 431,604 BTU
- Combined burner load
- 76,000 BTU/h
- Propane used per day
- 1.27 gal/day
Assumptions and limits
- Propane holds about 91,500 BTU per gallon (higher heating value) and weighs about 4.24 lb per gallon at 60 °F; both vary slightly with temperature.
- Appliances burn at their full rated input while lit. Grills and heaters on low burn less, so real run time is longer.
- The tank is run to empty. In practice regulators lose pressure near empty, and in cold weather a nearly empty tank may not vaporize enough gas.
- Cylinders are assumed full to their rated weight. Exchange cylinders are often filled to 15 lb instead of 20 lb.
Sources: US Energy Information Administration and DOE Alternative Fuels Data Center: propane heat content about 91,500 Btu per gallon NFPA 58 Liquefied Petroleum Gas Code: container filling limits (about 80% of water capacity for ASME tanks)
Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.
Questions
How long does a 20 lb propane tank last?
A full 20 lb cylinder holds about 4.7 gallons, or roughly 432,000 BTU. That runs a 36,000 BTU/h grill on high for about 12 hours, or an 18,000 BTU/h heater for about 24 hours.
How long will a 500 gallon propane tank last?
Filled to 80% it holds 400 gallons, about 36.6 million BTU. An 80,000 BTU/h furnace running 8 burner-hours a day uses 640,000 BTU, so the fill lasts about 57 days.
How many BTU are in a gallon of propane?
About 91,500 BTU per gallon, or roughly 21,600 BTU per pound. A therm (100,000 BTU) takes about 1.09 gallons of propane.
Why are large propane tanks only filled to 80%?
Liquid propane expands as it warms. The 20% vapor space keeps the tank from becoming liquid-full and opening its relief valve on a hot day.