It’s a common question asked by propane users, why isn’t my propane tank filled all the way? The answer is a preventative safety measure called the 80% fill rule.
Consider this: propane, like water, expands when heat is added. Propane, however, will increase in volume nearly 17 times greater than water over the same temperature increase. You can imagine this phenomenon because liquids take the shape of their container (in our case the tank) and volume rises as heat rises. If your tank was 100% full at any given time and the temperature inside the tank got a little hotter, your tank would become dangerous.
This is why propane tanks are filled to 80 percent. The same amount of propane may read differently at 80 percent on a mild March day than in mid-July. The extra space in the tank is a cushion against the pressure that builds up in a tank when it’s hot.
To calculate the fill capacity of your propane tank, multiply the tank’s total capacity by 0.8. For quick reference, here is a cheat sheet: