Propane is arguably the cleanest burning fossil fuel we can use in the house without worrying too much about smoke and carbon monoxide poisoning. Since it ships in metal propane cylinders/canisters, it is an easier alternative to natural gas lines for people living off the grid.
Propane cylinders come in different shapes and sizes. They’re all categorized by their weight. Some weight is from the canister itself, while the rest is from the propane.
It follows that if you know the canister’s weight, you can measure propane levels using weight – and some math.
To do this math, you need to know how much a gallon of propane weighs.
Propane Weight Per Gallon
As a liquefied gas, propane is highly susceptible to density chances. The ambient temperature is the biggest determinant of how dense your propane is (and, therefore, its weight per gallon).
As standard, anyone selling propane by weight measures at room temperature. This is 77°F. At this temperature, a gallon of propane weighs in at 4.11lb.
Propane will shrink when it’s cold and expand once the temperature increases. It will expand by roughly 1.5% for every 10°F temperature gain.
ProTip: The drastic changes in volume necessitate the 80 percent propane cylinder rule. Any propane canister can’t take more than 80% of its available volume on LPG. The remaining 20% creates room for expansion.
Propane Cylinder Weight Chart
|Ambient Temperature||Pounds Per Gallon (lb/gallon)|
How to Weight a Propane Tank
Weighing a propane tank should be simple if you have a weighing scale that goes high enough. The scale should measure slightly more than the predicted weight of your propane tank when it is full.
- Remove any attachments from the propane canister.
- Place your weighing scale on flat ground and zero it out
- Place the propane tank on the weighing scale and wait for the reading to settle
- Take at least three readings and average them if you want an accurate number. Otherwise, one reading should suffice.
You could also use a hanging scale to get the propane tank’s weight.
Since you are interested in how much LPG is in the cylinder and not the entire weight, including the metal canister, you will have to subtract the canister’s weight from the values you read on your scale.
How Much Does My Empty Propane Tank Weigh?
Mass-produced empty propane tanks should theoretically have similar weights in the category range. All 20lbs tanks should weigh the same when empty.
While this is a good starting average, it can’t be right at all times. The actual empty weight could deviate depending on:
- The manufacturing process
- The density of raw materials used in the batch
- The tear and wear on the propane tank
The most accurate way to know how much your tank weighs is by reading the stamped capacities. Propane tanks will have the WC (water capacity) rating and the TW (Tare Weight) capacity.
Tare Weight (TW) denotes the tank’s weight when it is empty. If your tank has a TW of 17lbs, it should weigh around 17lbs when empty and 27lbs when it has 10lbs of gas.
If you can’t find this reading, you can figure it out by measuring it when it is empty. You could also measure it when it is full to get a reference point and then subtract the weight of propane you filled up to get the tank’s weight.
ProTip: The variations in weight are often small, and most people will comfortably live with the slight differences.
How Much Do 7 Gallons of Propane Weigh?
You should expect 7 gallons of propane to weigh around 30 pounds at room temperature.
Here is the math
Pounds per gallon x number of gallons = Weight
4.2 x 7 = 29.4 pounds
The weight of the entire tank will exceed the 29.4 pounds + the tank’s empty weight.
How Many Gallons are in a 20lb Propane Tank?
With a tare weight of 17 pounds, a full 20 lb tank should weigh around 37 pounds. The 20 pounds of propane in the tank translates to 4.7 gallons of propane.
Gallons = Total Propane Weight / Propane Weight per Gallon
20/4.2 = 4.7 Gallons
ProTip: The math is the same for any tank size. You can calculate how many gallons of gas you have if you have the tank’s tare weight and the expected propane weight per gallon, depending on the temperature.
How to Calculate Remaining Propane Using Weight
Estimating how much gas you have remaining in your cylinder using a weighing scale should be easy if:
- You know the ambient temperature
- Know the tank’s tare weight
- Have a table to estimate gallons per weight depending on temperature (we shared one at the very start of this piece)
Here is what you should do:
- Remove any attachments from the tank
- Place the tank on a weighing scale and read its weight once the scale settles
- The weight of propane will be (The Weight You Read – The Tank’s Tare Weight)
- To get the volume of propane in gallons, Take the Propane’s Weight / Weight Per Gallon of Propane.
Here is an example of readings from a possible measurement reading. ( We are using a 20-pound tank and measuring it at room temperature (77F)
Propane Tank Tare Weight: 17 lbs
Weight of Tank and Propane: 26 lbs
Actual Propane Weight: 26-17 = 9 lbs
The volume of gas in Gallons: 9 / 4.11 = Approx 2.2 Gallons of gas
While using weight to estimate how much propane you have left works. Other alternatives would be:
Buying a Gauge
You could purchase an external gauge that automatically calculates the remaining volume or gives you the tank’s pressure. You can use this pressure to estimate how much gas you have in the tank.
- A pressure gauge will give you pressure readings. You can get one with estimated volumes printed on the scale or get a chart to tell you what different pressure levels mean
- A weight scale that lets you key in the tank’s TW as a preset and automatically gives calculated readings of the remaining propane weight.
- A digital scale that calculates remaining propane weight or volume. Some even account for ambient temperature when giving the readout.
Using the Warm Water Trick
If you have no tools or gauges on you and want a rough estimate of how much propane you have left in the tank, you can douse it in some water and feel the tank for a cool level of the tank.
The theory is that any propane remaining in the tank will absorb the heat from the warm tank, cooling down the metal surface. The slight differential between where there is propane and where the tank is empty should be discernable.
- Get some warm water in a bucket.
- Pour it over the side of the tank such that it drips to the bottom
- Run the tank along the side of the tank from top to bottom, looking for the transition area in temperature (from a bit warm to cooler)
- That is your approximate propane level
Is My Propane Tank Empty?
The only way to tell if your propane tank is empty is by using one of the above tricks to tell how much propane is remaining. I also find shaking the tank and listening to how much LPG sloshes around a good way to find out if you have anything left.
You will also tell when the tank is empty if none of your propane appliances ignite or run. While this could mean a valve is closed, there’s a blockage or a leak, coupling it with the shake, warm water trick, or gauge reading to see if there is something in the tank is a good way to figure out if the tank is empty.
Why Isn’t My Propane Tank 100 Percent Full?
If you use a gauge or the warm water trick, you will notice that the propane tank is never 100% full immediately after refilling it.
This is the 80% rule that dictates above-ground propane tanks should always leave at least 20% free space when full.
Check this too: Can a Propane Line Freeze?
Despite this space, you still have your money’s worth in propane. Manufacturers account for the extra space and rate their propane cylinders with the 80% cap in mind.
A 20-gallon tank will have 20 gallons of propane even though it is filled to 80%. It will weigh the 37 pounds it’s supposed to weigh at room temperature.
Is Propane Weight Always Consistent?
No. Propane weight varies depending on a couple of things. The most crucial is the propane batch’s ambient temperature and moisture content. Since most filling stations have underground tanks, the temperature shouldn’t be much of a worry when refilling. It will only come into play when storing your LPG in above-ground tanks.
Moisture content will also be within acceptable limits from the refinery and only vary if the distributor isn’t handling their propane properly.