General · Home & Outdoors
Propane tank run time.
Pick a tank, add what you burn and for how long, and see the hours and days of propane you have left.
Inputs
How long it lasts
Fill the form and press Calculate.
How this is calculated
Run time = usable propane energy ÷ the BTU per hour your appliances burn. Propane holds about 91,420 BTU per US gallon (higher heating value; the lower heating value is 84,250), per the US DOE Alternative Fuels Data Center fuel properties table, checked 2026-10-02. Pound-rated cylinders (1, 20, 30, 40 and 100 lb) are sized by propane weight at the fill limit, so they are used as stated: 20 lb ÷ 4.24 lb per gallon ≈ 4.7 gal ≈ 431,000 BTU. For gallon tanks (120, 250, 500 or custom) the page applies the 80% maximum fill to the nominal water capacity, as NFPA 58 limits fill to 80%. Nominal cylinder specifications come from US DOT 49 CFR Part 178. The 4.24 lb per gallon figure is a planning value for liquid propane near 60°F, used only to convert pounds to gallons.
“Current level” is a percent of a full tank. A tank gauge on a gallon tank reads percent of water capacity, so a freshly filled tank reads about 80%; enter 100 for that tank, or enter your gauge reading ÷ 80 × 100. Generators are entered as gallons of propane per hour (BTU/hr = gallons/hr × 91,420); use the figure on the generator’s fuel-consumption chart. Typical BTU ratings shown are examples, not sourced figures: replace them with your appliance’s label value. Burners rarely run at full rating all the time, so real run time is usually longer on a grill or heater turned low. The page does not model cold weather or the vaporization limit of a small tank under a large load, which can cut real output. Nothing you enter leaves your browser.
Frequently asked questions
How long will a 20 lb propane tank last?
A full 20 lb tank holds about 4.7 gallons, or roughly 431,000 BTU. On a 30,000 BTU/hr grill burning at full rating that is about 14 hours; at 10,000 BTU/hr it is about 43 hours. Enter your own BTU/hr above.
How long will a 100 lb propane tank last?
A full 100 lb cylinder holds about 23.6 gallons, roughly 2.16 million BTU, five times a 20 lb tank. A 40,000 BTU/hr heater running continuously would use it in about 54 hours.
Why does a tank only fill to 80%?
Liquid propane expands as it warms, so tanks are filled to at most 80% of their water capacity (NFPA 58) to leave vapor space. That is why a gallon-rated tank holds 0.8 times its nominal size.
How do I find my appliance’s BTU rating?
Look at the rating plate or manual, which lists input in BTU/hr. For a generator, use the manufacturer’s propane gallons-per-hour at your load.
How do I know how much propane is left in my tank?
Use the gauge if the tank has one. For a portable cylinder, weigh it and subtract the tare weight (TW) stamped on the collar from the total; the remainder is propane in pounds. Divide by the full capacity to get the percent to enter above.
Is the answer exact?
No. It is arithmetic on the numbers you enter. Burn rates change with settings, outdoor temperature and appliance condition, so treat the result as a planning estimate and keep a spare.