How many gallons of propane are in a 120-gallon tank? Some of you may recognize this as a trick question. The tank has a 96-gallon capacity. So why aren’t propane tanks ever filled up all the way? That has to do with the 80% fill rule.
The 80% fill rule is a preventative safety measure against the fluctuations that happen inside a tank. Propane, like water, will expand when heat is added to it. Propane, however, will increase in volume nearly 17 times greater than water over the same temperature increase. To allow for this expansion, propane containers are filled to only 80% of their capacity.
That means a tank that is 80% full on a mild March day, might register as 85-percent (or higher) at the mid-July cook out. It’s the same amount of propane, but it’s taking up more space. So the extra space in the tank is a cushion against the pressure that builds up in a tank when it’s hot.
Want to know the fill capacity of your propane tank? Multiply the tank’s total capacity by 0.8. Here’s a handy cheat sheet showing you total number of gallons remaining based on your tank size and current gauge level.