Propane Weight Per Gallon: How Much Does Gallon Of Propane Weigh?

1 gallon of water weighs 8.34 lbs. How much does 1 gallon of propane weigh? Some are shocked to learn that’s not 8.34 lbs; that figure is not even close to the real propane density.

When calculating the weight of full propane tanks, it’s always useful to know how much does a gallon of propane weigh. We’re talking about propane density.

The density of LPG (liquid petroleum gas) is expressed in pounds per gallon (lb/gallon). We’re going to take a 3 pronged approach here that should help illustrate how to use the propane density:

  1. First, we’ll look into how much does propane per gallon. Note: Propane density depends on the temperature. We will look at how does 1 gallon of propane weigh at 77°F and a few other temperatures.
  2. Secondly, we will compare the density of LPG (propane density) to water. Example: We know that the density of water is 8.34 lb/gallon. It’s about twice as heavy as propane.
  3. Thirdly, we will look into lb to gallon propane transformation. This will help us to answer this key question: How many gallons of propane are in a 20 lb tank? Here we also have to apply the 80% max. rule.
    how many gallons do 20 lb propane tank hold
    5 lb to 100 lb propane tanks hold anywhere between 1 gallon and about 20 gallons of propane.

Let’s start with the propane weigh per gallon:

How Much Does A Gallon Of Propane Weigh?

1 gallon of propane weighs 4.11 lb. That’s if you measure it at 77°F, of course.

It’s important to understand that the 4.11 lb/gallon density of LPG only applies at room temperature (77°F).

As you know, gases (even liquified gasses such as liquid propane) expand with higher temperatures and shrink with lower temperatures. That’s why when we quote how much does propane weigh per gallon, we also have to include the temperature.

In fact, according to Wikipedia, “Propane expands at 1.5% per 10°F.”

This also explains the reason why we need the 80% max. rule when filling up propane tanks. We will use this rule in the 3rd part when determining how many gallons does a 20 lb propane tank hold. Key part: A 20 lb propane tank can hold at most 16 lb of propane.

Here are some propane densities at different temperatures to illustrate that a gallon of propane doesn’t always weigh 4.11 pounds:

  • Propane density at 90°F: 4.05 lb/gallon.
  • Propane density at 80°F: 4.09 lb/gallon.
  • Propane density at 77°F: 4.11 lb/gallon.
  • Propane density at 70°F: 4.17 lb/gallon.
  • Propane density at 60°F: 4.20 lb/gallon.

Higher temperatures: Decrease the density of propane, increase propane volume, increase the pressure inside a propane tank.

Lower temperatures: Increase the density of propane, decrease propane volume, decrease the pressure inside a propane tank.

How Does Propane Density Compare To Other Liquids

It’s interesting to see how to light propane actually is. It only weighs 4.11 lb per gallon.

That, unfortunately, means that you will be able to fill a smaller mass of propane.

Example: Propane has a density of 4.11 lb per gallon. Water has a density of 8.34 lb per gallon.

This difference in density between propane and water means that you will be able to put about twice as much water (measured by mass) in a propane tank than actual propane.

Basically, if you fill up a 20 lb propane tank with water instead of propane, it will be heavier. By about 10 lb in total.

How Many Gallons Of Propane In A 20 Lb Tank?

Now that we know that one gallon of propane weighs 4.11 lb, we can calculate how many gallons of propane are in a 20 lb tank.

First of all, however, we need to take an important note. 20 lb propane tanks might be a 20 lb propane tank but it doesn’t hold 20 lb of propane. In fact, it only holds 16 lb of propane.

That’s because of the 80% rule when filling propane tanks. They are never filled up to 100% because, in the event of higher temperatures (house fire, for example), they might explode.

To figure out how many gallons of propane are in a 20 lb tank, we need to convert 16 lb of propane (amount of propane in 20 lb propane tank) to gallons. Here’s how that’s done:

20 lb Propane Tank Propane Content = 16 lb ÷ 4.11 lb Per Gallon = 3.89 Gallons

That means that a 20 lb propane tank holds 3.89 gallons of propane.

In this way, you can figure out how many gallons of propane do different propane tanks hold. To get the sizes right, you can check our article about propane tank sizes with a chart of all 48 propane tank sizes here.

If you can’t find an adequate answer to your question, you can pose one in the comments and we’ll try to help you out as best we can.

5 thoughts on “Propane Weight Per Gallon: How Much Does Gallon Of Propane Weigh?”

  1. Propane weighs 4.2 lbs per gallon at any temperature. The weight does not change. Also the 80% rule on DOT cylinders doesn’t apply the way it does on ASME tanks for home use. For portable cylinders you take the number stamped on the collar as water capacity and multiply it by 0.42 to get propane capacity in pounds or gallons (according to how the water capacity is listed in pounds or gallons). A 20 lb cylinder hold 20 lbs of propane at 80% capacity and that applies up through 100 lb cylinder sizes.

    • Unlike water , the density, which is mass per unit volume , of propane varies quite a bit with temperature. You have stated otherwise, which is not true.

  2. You have much good on here but are incorrectly telling people à 20# dot cylinder can’t hold 20#s. Not true at the 70 degrees F a 20#cylinder is 80% filled when it has 20 pounds.

    • Hi Frank, well, a 20 lb is physically capable of holding 20 lbs of propane. However, you always fill it up to 80% of its max. capacity. This is a preventative measure that is always followed.

      • A 20 pound tank has a water capacity of almost 6 gallons. At 80% fill using the propane sale temperature standard of 60 degrees fahrenheit this is 20 pounds. This is how dot cylinders rated in pounds including 20, 30, 100, and 420 pound cylinders are rated. When filling these a scale is preferred and the tare weight on the side of the cylinder is added to the rated weight to get the correct fill. OR the bleed screw is opened to know when it reaches the 80% mark. The bleed screw is attached to a tube that goes 20% down into the cylinder to so you know when the liquid reaches that level.


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