Have you ever checked the gauge on your propane tank right after a delivery from your propane company and noticed it only reads 80 % full? You may have thought, “What’s wrong here? I thought I was getting a fill-up?”

Don’t worry. There is nothing wrong. In fact, everything is exactly right. Your tank gauge never reads 100% due to safety reasons, or what is known in the propane industry as the 80/20 rule. Let’s explain.

How Propane Reacts Once It’s In Your Tank

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% capacity; underground tanks can be filled slightly higher because they are insulated against the heat.

That extra space in the tank provides a cushion against the pressure that builds up inside the tank. For example, a 500-gallon tank filled to 80% will safely hold 400 gallons of propane.

This safeguard is especially important in hot weather—when liquid propane will expand the most. It’s important to note that the amount of gas in the tank doesn’t actually change during periods of expansion and contraction–only its density does. For example, if you notice that the tank gauge reading fluctuates slightly during quick temperature swings (hot day, cool night), again, don’t worry because that’s perfectly normal.

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.

Propane Delivery Options

Your propane company will work closely with you to ensure you always have enough propane in your tank. They may even be able to put you on an automatic delivery schedule. This will significantly reduce the chance of running out of propane. And you won’t have to worry about checking your tank and then taking the time to call in a delivery request.

If you have any questions about your propane deliveries, please reach pout to your local propane company.