Propane Tank Levels: The 80% Rule

An image of a propane tank gauge on a propane tank.

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Have you ever found it odd that your fleshly refilled propane tank is only 80% full? As a first-time time propane user, this aspect can be disturbing. You might even think your gas supplier has scammed you. But that is not true. 

That is perfectly normal as it is the industry standard practice known as “the 80/20 rule”. The 80 percent rule also has a deeper meaning that touches on the scientific properties of propane. 
If this concept sounds a little confusing, worry not. In this article, we will explain everything you need to know about this rule. In addition, we will learn how many gallons of propane are in 80% of a tank, at what percent to fill propane tank, whether location affects the 80%rule, and more. Let’s jump right in!

What Does 80% Mean on a Propane Tank?

This question bugs most users’ minds, and it is related to the 80% rule. In other words, can you overfill a propane tank? Also known as the 80/20 propane rule, it is a safety industry standard when you want to fill a propane tank. 

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Most liquids expand at variable rates, but propane’s expansion rate is much higher than in most other liquids and gasses. In fact, the propane expansion rate is 17 times the expansion rate of water! 

And right there lies the issue. If the propane tank is overfilled to the maximum, the expanding gas can cause it to burst under pressure. To avoid this risk, industry experts have had to work around the problem. 

So can you overfill a propane tank? The answer is no. It is hazardous and propane dealers do not allow it. Instead, the standard has been to leave a 20% space of the tank capacity for this expansion.

Remember, this space is also essential for the vaporization of propane. You see, propane is heavily pressurized and that is how it turns into liquid. 

Once stored in the home-use tanks, propane has to convert to vapor for the appliances. And the 20% is the space where this vaporization happens. 

Keep in mind also that this expansion is highly sensitive to temperature fluctuations. If you check your propane tank during the hot weather, the gauge will likely show a reduced gas level. This should not be a cause for alarm, though. Your gas is not any less than it should be. 

It is because of the vaporization that has taken place above the propane level in the tank. Once the temperature lowers, propane condenses back to the usual level. So the amount of propane remains the same. This same phenomenon should be taken into account when thinking about how low can you let your propane levels to drop inside a tank.

What changed is the form of some of the propane from liquid to vapor, caused by the rise in temperatures, giving a misleading impression.
The temperature sensitivity aspect also accounts for the selective painting of the propane tanks. In case you haven’t noticed, all above-ground propane tanks are painted white instead of dark colors. White color reflects light, while dark colors absorb heat, increasing the internal temperatures of the tank.

How Many Gallons of Propane Is 80%?

We have sufficiently explained why are propane tanks only filled to 80. Now we want to know the fill capacity in gallons. Because yes, your tank is indeed holding 80% of its capacity. But how much propane is precisely, in the newly filled tank?

To get this figure, you will need to do some simple calculations. But before proceeding, note that the fill capacity does not depend on the tank size. In other words, it doesn’t matter whether you have a 200-gallon, 500-gallon, or 1000-gallon maximum capacity tank.

One of the common beliefs is that larger tanks require smaller space. For example, 20% of a 1000-gallon tank can look like a substantial unnecessary space. But the truth is, larger propane tanks actually require more space.  

Propane expands more in larger tanks than it does in smaller tanks. And so, it is an accepted industry standard to set a uniform percentage across the board.

Having dispensed with that, we can demonstrate with an example the fill capacity or the 80% capacity of a tank. Let us say you have a newly filled 20-gallon BBQ tank. To know how much propane is in the tank, multiply by 0.8 ( 80/100=0.8) x the tank’s maximum capacity.
In our case, the 80 percent capacity of the 20-gallon tank is 16 gallons. Using the same method, you can determine the amount of propane in any tank. For a 500-gallon tank, the amount comes to 400 gallons, the 1000-gallon fill capacity is 800 gallons, and so forth.

How Low Should I Get My Propane Tank Get Before Refilling?

Related to the question of why are propane tanks only filled to 80, is the 20 percent rule. You see, you cannot simply fill the propane tank and use it to the last drop. Doing that will be in violation of propane tank refill rules, and the consequences can be dire.

So what is the minimum amount to retain in the tank? The propane tank refill rules require to refill once the tank hits 20% of its capacity. Note that slightly going below this figure isn’t considered a big issue, really. But there are potential risks of flouting this rule entirely. Therefore, you ensure this doesn’t happen often.

Here are some of the potential risks.

Insufficient Pressure

Low propane level causes low pressure inside the propane tank. Consequently, most of your appliances will not work. Obviously, that means your family loses out on some of the conveniences and comforts. Here’s a complete guide on propane tank pressure if you want to read more.

Gas Leakage

In the worst-case scenario, the 20% percent can get depleted. If that happens, the system experiences leaks, which is a potential fire or explosion hazard.

Propane System Rusting.

Tank rusting compromises its structural integrity. If the tank develops weaknesses, it can explode. Visit this piece to read how long can a propane tank last and how to properly dispose of it in due time.

Unexpected Costs

The propane tank refill rules require the whole system to be re-inspected if the propane tank runs out. This is an extra cost that can dig deeper into the family budget. Visit these two pieces to check how much it costs to fill up a 20-lb propane tank and a 100-lb propane tank.

Does the Tank Location Affect the 80% Rule?

Where the tank is located affects the 80% rule. The volatility of the propane will be affected by the surrounding temperatures. Generally, underground tanks tend to be cooler than above-ground or garage ones.

Heat penetrates less below the ground resulting in lower temperatures cooling the tank. As a result, there is less expansion of propane inside the tank. Therefore, the 80% propane tank refill rule can be adjusted slightly upwards without any worries. 

In contrast, the above-ground tanks receive direct hits from the sun, causing the fluid inside to expand. Have you ever wondered why propane tanks are painted white? That is the reason. Unlike dark colors that absorb heat, white colors reflect off light, accounting for the less rise in temperature inside.

It is also common practice to reduce the temperature fluctuations inside the propane tank by providing sufficient insulation. You can, for example, build a shelter over the tank to provide shade against sunlight. 

However, be sure to consult technicians from your gas provider first. You do not want to build a shelter that can potentially create more risks.

Final Thoughts

In conclusion, can you overfill a propane tank. No propane gas supplier would recommend it. It is clearly against the 80% propane tank rule which is an established standard used by all reputable propane gas dealers. 

As we mentioned, the purpose of this rule is to cater to the fast expansion of propane due to temperature changes. So next time your newly filled propane tanks indicate 80% on the gauge, know it wasn’t a mistake. 

It is normal industry practice to prevent propane tank overfilled, to allow space for the vaporization of propane, and to safeguard your safety.

Check More Interesting Reads

Understanding Parts of a Propane Tank
Does Propane Go Bad? Shelf Life & Storage Tips
How To Tell If Your Propane Tank Has Expired
Does My L/P Tank Need to Be Re-certified?

Ryan McCabe

Ryan McCabe

Ryan is a home and small business energy expert, having overseen retail propane, heating oil, and HVAC services companies. On ComparePropane, Ryan writes about things that people should know when shopping for or using propane in their homes and businesses. 

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