How Many Pounds of Propane Are in a 100-Gallon Tank?
By windflash · Published Oct 18, 2026 · Updated Oct 18, 2026
A 100-gallon propane tank holds about 80 gallons of propane at the standard 80% fill, which weighs about 339 pounds (80 gallons × 4.24 lb per gallon). Filled to 100%, it would hold about 424 pounds, but tanks are not filled that full.
The math
- Water capacity: 100 gallons.
- Safe fill: tanks are filled to 80% to leave room for expansion, so 100 × 0.8 = 80 gallons.
- Weight: liquid propane weighs about 4.24 lb per gallon, so 80 × 4.24 = 339.2 lb.
Propane's density changes a little with temperature; 4.24 lb per gallon is the common figure for about 60°F.
100-gallon tank vs 100-pound cylinder
These are often confused. A 100-pound cylinder holds about 23.6 gallons of propane. A 100-gallon tank holds 80 gallons, more than three times as much.
What does it cost to fill a 100-gallon tank?
At the U.S. average of $2.64 per gallon (EIA, week of Oct 12, 2026), 80 gallons costs about $211.04 in fuel. Prices vary a lot by state; see propane prices per gallon by state.
How long will it last?
Run time in hours = gallons × 91,452 BTU per gallon ÷ the appliance's BTU/h rating. As an example, an appliance rated at 40,000 BTU/h running nonstop burns about 0.44 gallons an hour, so 80 gallons lasts about 183 hours. Check your appliance's nameplate for its real rating; most appliances do not run continuously.
Gallons to pounds converter
Other tank sizes
| Tank | Propane (80% fill) | Weight |
|---|---|---|
| 20 lb cylinder | 4.7 gal | 20 lb |
| 30 lb cylinder | 7.1 gal | 30 lb |
| 40 lb cylinder | 9.4 gal | 40 lb |
| 100 lb cylinder | 23.6 gal | 100 lb |
| 100 gal | 80 gal | 339 lb |
| 120 gal | 96 gal | 407 lb |
| 250 gal | 200 gal | 848 lb |
| 500 gal | 400 gal | 1,696 lb |
| 1000 gal | 800 gal | 3,392 lb |
Full chart with cost to fill: propane tank size chart. Compare heating costs in the propane vs electric heat calculator.
Frequently asked questions
How many pounds of propane are in a 100 gallon tank?
About 339 pounds at the standard 80% fill: 80 gallons × 4.24 lb per gallon.
How many gallons of propane are in a 100 pound cylinder?
About 23.6 gallons (100 lb ÷ 4.24 lb per gallon). A 100-pound cylinder is much smaller than a 100-gallon tank.
Why does a 100 gallon tank only hold 80 gallons?
Propane expands when it warms, so tanks are filled to about 80% of their capacity to leave room for expansion.
How much does it cost to fill a 100 gallon propane tank?
At the U.S. average of $2.64 per gallon, 80 gallons costs about $211.04 in fuel, before delivery fees and taxes.