Why Doesn’t My Propane Tank Read 100% Full?

If you’re a keen observer, you’ve probably noticed that the gauge on your propane tank never reads 100%–even right after you’ve received a delivery from your propane company. But that’s strictly by design and for safety purposes. Here’s why.

The propane in your tank is stored as a liquid. The liquid changes to gas before it leaves the tank. That’s why it’s called liquid petroleum gas (LPG).

Like any liquid, propane will expand when its temperature rises. The difference with propane is that it expands a lot, and quickly –its volume increases nearly 17 times the volume of water over the same temperature increase.

This is why your 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 percent capacity; underground tanks can be filled slightly higher because they are insulated against the heat. The extra space in the tank provides a cushion against the pressure that builds up in a tank

As an example, a 500-gallon tank filled to 80% 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. If you notice that the tank gauge reading fluctuates during quick temperature swings (hot days, cool nights), don’t worry, that’s perfectly normal. Also 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 also a reason why you should never paint your outdoor propane tank a dark color, since dark colors absorb more heat.

Reading your tank gauge

Most propane tanks are equipped with a gauge, located under the dome. This provides the approximate percentage of fuel in the tank. If the gauge reads 30% or less, you should arrange for a delivery from your local propane company.