Why Your Propane Gauge Never Reads 100%

Understanding the 80/20 Rule for Propane Tanks

If you’re someone who keeps a close watch on their propane tank gauge, you may have noticed that when your propane supplier delivered your fuel, they didn’t fill your propane tank all the way up to the 100% level.

Don’t worry. There is nothing wrong. This is known in the industry as “the 80/20 rule,” and it’s done for a very important reason: safety.

Propane: From Liquid to Gas

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 propane 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. That can become a hassle during periods of bitter cold temperatures.

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