How Many Gallons in a 20 lb Propane Tank?
A standard 20 lb propane grill tank can be confusing because the tank size is listed by propane weight, while many people want to know how much fuel that equals in gallons. It also gets confusing because a refilled 20 lb tank and an exchanged 20 lb tank may not contain the same amount of propane.
Quick Answer: A properly filled 20 lb propane tank holds about 4.7 to 4.8 gallons of propane. Many exchange tanks contain only 15 lb, which is about 3.6 gallons.
| Tank situation | Propane weight | Approx. gallons |
|---|---|---|
| Properly filled 20 lb tank | 20 lb | About 4.7 to 4.8 gallons |
| Common exchange tank | 15 lb | About 3.6 gallons |
The math is simple: propane weighs about 4.2 pounds per gallon, so 20 pounds of propane works out to roughly 4.7 to 4.8 gallons.
| Tank Size | Propane Weight When Full | Approximate Gallons of Propane |
|---|---|---|
| 20-lb tank | 20 lb | 4.7–4.8 gallons |
| 30-lb tank | 30 lb | 7.1 gallons |
| 40-lb tank | 40 lb | 9.5 gallons |
| 100-lb tank | 100 lb | 23.6 gallons |
How this works: Propane weighs about 4.2 pounds per gallon, so dividing the tank’s propane weight by 4.2 gives the gallon estimate.
Why Are Propane Exchange Tanks Only 15 lb?
Many propane exchange tanks are not filled with the full 20 pounds of propane. Common exchange tanks often contain about 15 pounds instead, which works out to roughly 3.6 gallons.
The reason is mostly business-related. The exchange company can sell the convenience of a ready-to-swap propane tank while supplying less propane than a true 20-pound refill. The tank itself is still a standard 20 lb propane cylinder, but the amount of propane inside may be only 15 lb.
That is why a propane exchange can be convenient, but not always the best value. If you want the most propane for your money, a refill station that fills the cylinder by weight or by gallon may give you closer to the full 20 pounds.
For example, Blue Rhino states that its propane grill exchange tanks contain a net weight of 15 pounds of propane. That is about 3.6 gallons, not the 4.7 to 4.8 gallons you would expect from a full 20 lb propane cylinder.
How Much Should a Full 20 lb Propane Tank Weigh?
A full 20 lb propane tank usually weighs about 37 to 38 pounds total, depending on the empty tank’s tare weight. Most empty grill tanks weigh around 17 to 18 pounds. To check yours, look for the TW stamped on the tank collar, then add 20 pounds of propane.
Why Doesn’t a 20-lb Propane Tank Hold a Full 5 Gallons?
It seems like it should, after all, propane weighs about 4.2 pounds per gallon, and 20 ÷ 4.2 is roughly 4.8 gallons. So why not just call it 5 gallons?
The reason is safety and temperature expansion.
Propane expands as it warms. If a tank were filled completely full of liquid propane, even a modest temperature increase could cause dangerous pressure buildup inside the cylinder.
Factoid: The pressure inside a propane tank is affected by temperature. It can range from about 60 psi (30 degrees F) to about 200 psi (100 degrees F).
To prevent this, propane tanks are designed with built-in expansion space. When a 20-lb tank is properly filled, the liquid propane occupies about 80% of the tank’s internal volume, leaving 20% empty space for the propane to safely expand.
Because of this safety margin, a “full” 20-lb propane tank ends up holding about 4.7 to 4.8 gallons, not a full 5 gallons.
So when people say a grill tank holds “about five gallons,” that’s just a rounded estimate — the real usable amount is a little less.
Why a Propane Tank Gauge May Not Show Full on an Exchange Tank
Many hanging propane tank scales are based on a standard 20 lb propane cylinder filled with 20 pounds of propane. If you use an exchange tank that contains only 15 pounds, the scale may not read “full” even though the exchange tank is newly swapped.
That does not necessarily mean the gauge is broken. It may be measuring the tank weight correctly, but using a true 20 lb fill as the “full” reference point. A fresh 15 lb exchange tank may show closer to three-quarters full on many hanging scales.
| Hanging scale reading | True 20 lb refill | 15 lb exchange tank | ~ gallons |
|---|---|---|---|
| Full | 20 lb | Not typical for a 15 lb exchange tank | 4.8 gallons |
| 3/4 | 15 lb | Fresh/full exchange tank | 3.6 gallons |
| 1/2 | 10 lb | About 2/3 of exchange fill | 2.4 gallons |
| 1/4 | 5 lb | About 1/3 of exchange fill | 1.2 gallons |
| Empty | 0 lb | Empty | 0 gallons |
Best Propane Tank Gauge for a BBQ Grill Tank
A propane tank gauge is one of the easiest ways to check how much propane is left in a 20 lb BBQ grill tank, especially if you do not want to weigh the cylinder.
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→ Flame King Propane Tank Digital Scale
→ Propane Tank Gauge Scale
Propane Tank Overfill Protection
Modern refillable grill-size propane cylinders are required to have an OPD, or Overfill Protection Device.
The OPD uses a float valve that shuts off filling when propane reaches the safe liquid level inside the tank (about 80% of internal volume).
How do I know if my propane tank has an OPD overfill protection device?
If you open the tank’s valve and propane comes right out, you do not have an OPD. They are also recognizable by the triangular hand wheel at the top of the valve itself. The hand wheel connecting to the valve stem is tamperproof and is not interchangeable with a cylinder not equipped with the overfill prevention safety mechanism.
Propane Weight versus Gallons
| Pounds of Propane | Gallons of Propane |
|---|---|
| 4.2 lb | 1 gallon |
| 8.4 lb | 2 gallons |
| 12.6 lb | 3 gallons |
| 16.8 lb | 4 gallons |
| 21.0 lb | 5 gallons |
| 25.2 lb | 6 gallons |
| 29.4 lb | 7 gallons |
1. Weigh your propane tank
2. Subtract its tare weight, marked “TW” on the tank collar.
3. Read chart above to find equivalent gallons.
Factoid: A standard 20 lb propane tank will typically fill safely with 20 pounds of propane because there is a built-in 80% safety margin in the tank design volume.
Most Accurate Way to Check Propane
There are a few easy ways to check your propane level. A quick gauge for convenience, or a scale for exact measurement.
Want the Most Accurate Reading?
The most precise way to know how much propane is left is to weigh the tank and subtract the tare weight (TW) stamped on the collar.
A small digital luggage scale works great for this and is easy to store with your grill gear.
Compact digital hanging scale (great for weighing propane tanks)
Etekcity Luggage Scale
→ Check price & availability(Amazon)
Propane Gallons FAQ
A properly filled 20-lb propane tank holds about 4.7 to 4.8 gallons of propane. Propane weighs roughly 4.2 pounds per gallon, and a full tank contains 20 pounds of propane by weight.
Not quite. A 20-lb tank holds slightly less than 5 gallons — typically about 4.8 gallons. Tanks are only filled to about 80% of their internal volume to allow room for propane expansion.
Many propane exchange tanks contain about 15 pounds of propane, or roughly 3.6 gallons, instead of the full 20 pounds, or about 4.7 to 4.8 gallons. This is mostly a business/pricing decision, not because a standard 20 lb propane cylinder cannot safely hold 20 pounds when properly filled.
If a full tank holds about 4.8 gallons, then a half-full tank contains approximately 2.4 gallons of propane.
Divide the propane weight by 4.2.
Example: 10 pounds ÷ 4.2 = about 2.4 gallons.
Want to know exactly how much propane is left in your tank?
👉 How To Tell How Full (or Empty) Your Propane Tank Really Is
The above statement – “The pressure inside a propane tank is affected by temperature. It can range from about 60 psi (30 degrees F) to about 200 psi (100 degrees F)” appears incorrect. If calculated, be sure to use the correct units of measurement when applying the ideal gas laws.
Ken, this one was really informative, I didn’t really know there was a difference between exchange and refilled tanks.