A 40 lb propane tank holds 40 lb of propane, right? *Not exactly.* The correct answer is **32 lb**, in accordance with the NEC guidelines. This is just one thing we need to clear up before we address how many gallons of propane are in a 40 lb propane tank.

Namely, this is the problem we have when trying to figure out how many gallons does a 40 lb propane tank hold:

- 40 lb or 40 pounds is a
*unit of*.**mass** - A gallon is a
*unit of*.**volume**

Because pounds and gallons are not measuring the same quantity, we have issues with figuring out how many gallons of propane go into a 40-pound propane tank.

To resolve this, we have to find a formula that *connects the mass* of forty lb of propane (it’s actually 32 lb, we will explain why further on) and the *volume of propane*, measured in gallons. This formula is called the propane density formula, and looks like this:

**Propane Density = Mass Of Propane (lbs) / Volume Of Propane (gallons)**

Now, the key metric we need to know to calculate how many gallons of propane you can fill into a 40 lb propane tank is propane density.

Let’s first check how much a gallon of propane weighs. After that, we will explain why you need to put only 32 lb of propane in a 40 lb propane tank:

## How Much Does A Gallon Of Propane Weigh (For 40 lb Propane Tank)

The density of propane is about 1/2 that of water; it is about 50% lighter than water. Water has a density of 8.34 lb/gallon at 77°F, for reference.

The density of propane in a 40-gallon tank depends on temperature. At 90°F, the density of propane is low (4.05 lb/gallon). At 60°F, the density of propane is high (4.20 lb/gallon). When filling gallons of propane into a 40 lb propane tank, we usually take the propane density at room temperature (77°F).

The key metric we need to calculate how many gallons of propane are in a 40-pound propane tank is that the median density of propane is **4.11 lb per gallon**.

Knowing this, we can simply calculate how many gallons of propane does a 40 lb propane tank hold. This is what the propane gallon equations for a 40 lb propane tank look like:

**40 lb Propane Tank (Gallons) = (0.8 × Capacity Of Tank (In lb)) / Propane Density (In lb/gallon)**

Now, we know that we have a 40 lb propane tank (this is the tank capacity) and that propane density is 4.11 lb/gallon. We also see that *‘0.8’* factor in there.

This is the **80% NEC propane tank rule** that states that propane tanks can be filled only to 80% (hence 0.8) of their capacity. That means that we can put at most 32 lb of propane in a 40 lb propane tank.

Let’s put in the numbers are calculate how many gallons of propane you can fill in a 40 lb propane tank:

**40 lb Propane Tank (Gallons) = **(0.8 × 40 lb) / 4.11 lb/gallon** = 7.79 Gallon Of Propane**

Here you have it: You can fill * 7.79 gallons *of propane in a 40 lb propane tank.

That is because 32 lb of propane (that goes into a 40 lb propane tank) has a volume of 7.79 gallons.

Given that the amount of gallons that goes into a propane tank varies a bit with temperature (propane density between 4.05 and 4.20 lb/gallon at temperatures between 60°F and 90°F), we can practically say that a 40 lb propane tank holds **about 8 gallons of propane**.

Hopefully, you now have a better understanding of how to figure out how many gallons are in a 40-pound propane tank, or any other tank for that matter. If you have any questions regarding these propane tank pounds vs gallons conversions, you can use the comments below and we will help you out.