A 20-Pound Propane Tank Holds About 4.7 Gallons
A full 20-pound propane tank holds about 4.7 U.S. gallons. Check exchange fill weights, understand the 80% rule, and calculate fuel remaining.
A standard 20-pound propane tank holds about 4.7 U.S. gallons of liquid propane when filled with 20 pounds of fuel. That is its propane capacity—not the weight of the steel cylinder. Cylinder manufacturer Flame King lists the capacity as about 4.7 gallons.
For outage planning, check the net propane weight on an exchange tank’s label before counting on that much fuel. As of October 2026, Blue Rhino says its grill tanks contain 15 pounds of propane, equivalent to about 3.6 gallons.
Pounds-to-gallons conversion
The Propane Education & Research Council lists liquid propane’s approximate weight as 4.2 pounds per U.S. gallon at 60°F. Use that for quick estimates; volume changes with temperature. Source: PERC Technical Pocket Guide, Table 1A.
Estimated gallons = pounds of propane ÷ 4.2
| Propane actually in the cylinder | Approximate U.S. gallons |
|---|---|
| 20 lb | 4.7–4.8 |
| 15 lb | 3.6 |
| 10 lb | 2.4 |
| 5 lb | 1.2 |
The rounded conversion gives 20 ÷ 4.2 = 4.76 gallons. That is consistent with the commonly stated capacity of about 4.7 gallons; neither figure is an exact volume at every temperature.
Does the 80% fill rule reduce it to 3.8 gallons?
No—do not multiply the 4.7-gallon figure by 80% again. It describes the amount of propane a standard 20-pound cylinder holds, not the volume of its entire interior.
Propane containers leave expansion space because liquid propane expands as it warms. AmeriGas explains the 80% fill rule. The distinction is between the container’s total internal volume and its rated fuel capacity: the approximately 4.7-gallon propane capacity already allows for that space.
How much is left in your tank?
For a portable cylinder, use its stamped tare weight (TW) rather than assuming every tank weighs the same:
Propane remaining, lb = total scale weight − stamped TW
Propane supplier Hocon Gas explains this weighing method. For example, a cylinder weighing 27 pounds with a TW of 17 pounds contains about 10 pounds of propane—roughly 2.4 gallons. See how to check whether a propane tank is empty for more detail.
If the fuel is for a backup generator, estimate runtime from the actual fuel remaining and the manufacturer’s consumption rating at your expected load. A 15-pound fill has 25% less fuel than a 20-pound fill, so do not use a full-20-pound runtime estimate unchanged. Use our guide to generator propane consumption for the next calculation.