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Propane Tank Calculator: Percent to Gallons and Days Left
Read your tank's gauge (or weigh your cylinder), choose the tank size, and see how many gallons you have, how much a fill will take, and how many days until you should call for a delivery.
How propane tanks are measured
Propane containers are sized two different ways, and mixing them up causes most of the confusion.
Stationary tanks (120, 250, 500 and 1000 gallon) are named for their water capacity, the total volume of the tank. The float gauge under the lid reads a percentage of that total volume. So the math is simple: gallons = reading × tank size. A 500 gallon tank at 45% holds 225 gallons.
Cylinders (20, 30, 40 and 100 lb) are named for the weight of propane they hold when properly filled. Most have no gauge, and the screw-on pressure gauges sold for grills mostly tell you whether there is any propane left, not how much, because cylinder pressure depends on temperature, not on how full it is. Weighing is the reliable method.
The 80% fill rule
Liquid propane expands noticeably as it warms, much more than water. To leave room for that, stationary tanks are filled to about 80% of their volume, and cylinders have an overfill prevention device (OPD) valve that stops the fill at the right level. In practice:
| Tank | Propane when full | Weight of propane | Energy |
|---|---|---|---|
| 20 lb cylinder | 4.7 gal | 20 lb | 431,000 Btu |
| 30 lb cylinder | 7.1 gal | 30 lb | 647,000 Btu |
| 40 lb cylinder | 9.4 gal | 40 lb | 863,000 Btu |
| 100 lb cylinder | 23.6 gal | 100 lb | 2.16 million Btu |
| 120 gal tank | 96 gal | 407 lb | 8.8 million Btu |
| 250 gal tank | 200 gal | 848 lb | 18.3 million Btu |
| 500 gal tank | 400 gal | 1,696 lb | 36.6 million Btu |
| 1000 gal tank | 800 gal | 3,392 lb | 73.2 million Btu |
Computed with 4.24 lb per gallon and EIA's 91,452 Btu per gallon. The fill level can be a little above or below 80% depending on temperature on fill day.
How long will my propane last?
The days-left figure needs your daily use. The best source is your own history: divide the gallons on a delivery ticket by the days since the previous fill. If you have no history, the default of 3.4 gallons a day comes from EIA's 2025–26 Winter Fuels Outlook, which expected an average Northeast home heating with propane to use 512 gallons from November through March (151 days). EIA's Midwest average works out to 4.2 gallons a day and the South to 2.4. These cover the whole house (furnace plus water heater, range and dryer if they run on propane), averaged over a winter, so expect more on the coldest days and less in shoulder months.
For a single appliance, such as a heater, grill or fire pit, use the propane runtime calculator. For a standby generator during an outage, use the generator runtime calculator; a generator can burn more propane in a day than a whole house does in a week.
When to order
Most suppliers ask for a call at 20 to 30%. Below about 20% you are close to a run-out. If a tank does run empty, the supplier generally has to leak-check the system before turning the gas back on, which can mean a service charge and a wait. If you are on will-call delivery in winter, set the reorder level to cover at least a week of your heaviest use.
Propane safety: if you smell gas (a rotten-egg odor), leave the area immediately, don't operate switches or phones near the smell, and call your propane supplier or 911 from a safe distance.
Frequently asked questions
How many gallons are in a 500 gallon propane tank at 50%?
250 gallons. Tank gauges read a percentage of the tank's total (water) capacity, so multiply the reading by 500. A 500 gallon tank is filled to about 80%, or 400 gallons, at most.
Why won't my propane company fill my tank to 100%?
Liquid propane expands a lot as it warms. Tanks are filled to about 80% so there is vapor space for that expansion. That's why a 'full' 500 gallon tank holds about 400 gallons and a full gauge reads around 80%.
How many gallons are in a 20 lb propane tank?
About 4.7 gallons. Propane weighs about 4.24 pounds per gallon, so 20 lb ÷ 4.24 = 4.7 gallons. Cylinder sizes are already stated as the weight of propane they hold when properly filled, so you don't apply the 80% rule again.
How do I check how much propane is in a 20 lb tank?
Weigh it. Subtract the tare weight stamped on the collar (marked TW, usually around 17 lb) from the scale reading. What's left is the propane in pounds. A full tank weighs about 37 lb. Pouring warm water down the side and feeling for the cold line also works roughly.
At what percentage should I order propane?
Most propane suppliers ask you to call at 20 to 30%. In winter, or if deliveries in your area take several days, aim for the higher end.
Sources
- Propane Education & Research Council, Propane Technical Pocket Guide: 91,502 Btu per gallon and 21,548 Btu per pound (ratio 4.24 lb/gal); 4.2 lb per gallon at 60°F.
- U.S. Energy Information Administration, Energy units and calculators: propane 91,452 Btu per gallon.
- U.S. Energy Information Administration, Winter Fuels Outlook 2025–26: propane household consumption November–March (Northeast 512, Midwest 639, South 365 gallons).
- AmeriGas, Propane tank sizes, and Ferrellgas, Why your propane tank is never full: water-capacity sizing and the 80% fill level.
Last reviewed October 25, 2026. Estimates only; see the disclaimer.