Why a Propane Tank Never Reads 100% Full

The propane in your tank is a liquid, which changes to a gas before it leaves the tank. Like any liquid, propane will expand when its temperature rises. Propane will actually expand 17 times the volume of water given the same temperature increase.

That’s why our delivery driver needs to leave extra space in your tank to allow for propane to safely expand. Aboveground propane tanks are typically filled to about 80% capacity; underground tanks can be filled slightly more, because they are insulated against the heat. The extra space in the tank provides a cushion against the pressure that builds up inside. As an example, a 500-gallon tank that is 80% filled will safely hold 400 gallons of propane.

This so-called 80% rule is especially important in hot weather — when liquid propane will expand the most. Keep in mind that the amount of gas in the tank doesn’t actually change during periods of expansion and contraction — only its density does.

Propane gas expansion is the reason you should never paint your outdoor propane tank a dark color, since dark colors absorb more heat.