Reading Your Propane Tank Gauge

Date: June 20, 2023

Understanding the 80/20 Safety Rule

checking propane supply tennessee With the cold weather months behind us in the Volunteer State, you’re most likely using much less propane now, especially if you rely on a propane gas furnace to keep your home warm.

Still, you never want to suffer the inconvenience of being without this versatile, American-made residential fuel. Homes in Nashville, Franklin, Townsend and many other communities depend on propane for a range of appliances, including:

  • water heaters
  • stoves and cooktops
  • clothes dryers
  • pool and spa heaters
  • fire pits
  • hearth products
  • grills and outdoor kitchens
  • backup generators

Your Propane Tank and the 80/20 Safety Rule

If you’ve ever checked your tank gauge right after getting a propane delivery, you may have noticed that your propane tank gauge doesn’t register all the way up to the 100% level. But there is a reason for this. It’s known in the industry as the 80/20 rule (or 80% rule), and it’s done for a very important reason: safety.

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

The difference with propane is that it expands a lot, and quickly –in fact, its volume increases nearly 17 times the volume of water over the same temperature increase.

So, once it’s inside your tank, propane will expand. To make sure there is room for that expansion to happen safely, aboveground tanks will be left at a capacity of about 80% full.

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. 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 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.

By the way, 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.

A Primer on Reading Your Tank Gauge

The gauge is on or near the top of your tank, usually located under the dome, and it resembles a meat thermometer.

The reading on the gauge 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 Tennessee propane company.

Keep in mind that the numbers you see on the gauge represent the percentage of propane in your tank—not the actual number of gallons. When your gauge level reaches about 30%, it’s time to call for a delivery. That ensures that there’s enough time for your propane supplier to get to you before you run out.