Why Your Propane Tank Is Never Full

March 9, 2022 –

When your propane tank is “full,” it’s intentionally never truly filled to capacity.

It’s because reliable propane companies like Paraco know to leave room for propane expansion.

Propane expands when it meets heat. In fact, propane tanks typically expand to nearly 17 times the volume of water even when heated to the same temperature. This happens naturally as a result of the environment and weather.

So to allow for this natural expansion due to temperature changes, above-ground propane tanks and propane cylinders commonly known as “BBQ tanks” are filled to only about 80 percent. (The amount of propane that underground tanks can be filled with is slightly higher, since they’re insulated from large temperature fluctuations by the soil.)

Leaving that 20 percent of extra space in the tank is protective and preventative. Without leaving that space, when the temperature inside the tank rises, there’s a risk for danger.

The 20 percent of free space ensures that even if the propane tank gets hot, the propane inside has room to expand. This keeps it from creating so much pressure that its volume exceeds the tank’s capacity. Our drivers use a fixed liquid level tank gauge to make sure we don’t overfill.

To calculate the approximate fill capacity of your propane tank and see how many gallons of propane it can hold at “full,” multiply the tank’s total capacity by 0.8.

If you have any other questions about propane usage and safety, Paraco is here for you. We’ll help with all of your propane delivery needs, whether for your home or business, so get in touch with us when you’re ready.


  1. Gale Clinton on December 19, 2020 at 10:08 am

    My acct. says the usable capacity of my tank is 260 gallons, I thought a 500 gallon tank had a 400 gallon usable capacity?

    • Isousa on January 5, 2021 at 8:45 am

      Hello Gale! We checked out your account and the size of your propane tank is 320 gallons, with the 80% rule the usable capacity is 260 gallons!

  2. Lily Bridgers on December 1, 2023 at 5:27 pm

    You know what’s going on in my world? The amount of backyard barbecues has increased, so I’m searching for a propane tank supplier. I really want to be a grill expert, and to keep the fire going for those amazing summer cookouts, I need a trustworthy dealer. I’ll simply remember that heat causes propane to expand. In fact, even when heated to the same temperature, propane tanks usually expand to roughly 17 times the capacity of water.

    • Michael Murphy on January 20, 2024 at 8:40 am

      Don’t use one of the tank-swapping outlets. They charge way too much. Buy a good quality portable cylinder (or two — you don’t want to run out in the middle of a big cookout). The 20-lb(5-gal) is the standard size, but a 30-lb will last longer. They can be filled at a good hardware store (most Tractor Supply stores sell propane by the gallon). I fill my 20-lb cylinders at my local welding supply shop, and save anywhere from $5 to $8 over swapping them out. Depending on how much you use, buying your own cylinders will pay off in a couple of years.

  3. jane on February 5, 2024 at 10:48 am

    How low can a 1000gallon backyard L P tank get before it does not let our hot water heater work ?

    • Ines on February 6, 2024 at 9:31 am

      Hi jane! The minimum level a 1000-gallon backyard LP tank can reach before affecting the operation of your hot water heater depends on various factors, including the distance from the house, length/size of lineset, and size of the heater. Generally, it’s advisable to contact your propane provider at no less than 20% capacity to ensure consistent performance until a delivery can be made.

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