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Propane Tanks & the 80% Fill Rule

How many gallons of propane are in a 120-gallon tank? Some of you may recognize this as a trick question. The tank has a 96-gallon capacity. So why aren’t propane tanks ever filled up all the way? That has to do with the 80% fill rule.

The 80% fill rule is a preventative safety measure against the fluctuations that happen inside a tank. Propane, like water, will expand when heat is added to it. Propane, however, will increase in volume nearly 17 times greater than water over the same temperature increase. To allow for this expansion, propane containers are filled to only 80% of their capacity.

That means a tank that is 80% full on a mild March day, might register as 85-percent (or higher) at the mid-July cook out. It’s the same amount of propane, but it’s taking up more space. So the extra space in the tank is a cushion against the pressure that builds up in a tank when it’s hot.

Want to know the fill capacity of your propane tank? Multiply the tank’s total capacity by 0.8. Here’s a handy cheat sheet showing you total number of gallons remaining based on your tank size and current gauge level.

80% tank level

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  1. Gary
    Posted June 7, 2015 at 10:49 am | Permalink

    Since most bigger propane tanks are horizontal cylinders, I find it hard to believe it is a linear correlation between level and gallons. Also since propane is used as a vapor, you need tank space to hold enough vapor for immediate use. This is also a significant factor in how full the tank is filled

  2. Pip
    Posted January 7, 2016 at 12:50 pm | Permalink

    I have a 100 gall (I think) tank. At what reading on the dial should I request a refill?? Not too early, since I want to meet the minimum order requirement. I only use it for a fireplace, and it can take 3-5 days to fill it after requested.

  3. Lori Clark
    Posted February 19, 2016 at 10:10 pm | Permalink

    How can they put 311.2 gallons of gas in my 3 100 gallon tanks when your chart show that the 80% fill rule a tank can hold 96 gallons. 96 gal times 3 tanks only comes to 288 gallons. how did they put 311.2 in them? Please explain.

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