The name “20 lb tank” refers to the fuel’s net weight, not the total weight of the cylinder when you lift it. That distinction matters more than most grill owners realize. Understanding the difference between fuel weight, volume, and gross weight helps you know exactly what you’re buying — and whether that exchange cylinder you just grabbed is truly full.
Why the “20 lb” Is Fuel Weight, Not Total Weight
Propane cylinders are rated by the net weight of the fuel they’re designed to hold, not the weight of the metal around it. A standard U.S. 20 lb cylinder holds 20 pounds of propane by weight. When full, that propane occupies about 4.6 to 4.8 gallons of liquid volume, according to manufacturer specifications from suppliers like Eagle Propane Sales.
Here’s where the confusion starts: an empty steel 20 lb cylinder typically weighs between 16.6 and 18.5 pounds. That weight, called the tare weight (TW), is stamped directly on the cylinder’s collar. So the math works like this:
- Empty tank (tare weight): roughly 17–18 lb
- Propane fuel: 20 lb
- Full tank (gross weight): approximately 37–40 lb
When you pick up a full cylinder, you’re lifting close to 40 pounds, not 20. If a tank feels noticeably lighter than that, it’s probably not full.
How Many Gallons Is 20 lb of Propane?
That figure assumes a proper fill to the cylinder’s rated capacity. A tank is never filled completely to the brim — propane expands significantly with temperature, so cylinders are filled to about 80% of their liquid volume to leave room for expansion. The 20 lb net weight rating already accounts for that safety margin.
The Exchange Cylinder Problem: 15 lb vs. 20 lb
Here’s the catch that catches most people. You’re paying for a “20 lb tank,” but you’re often getting 15 pounds of fuel inside it.
Tare weight values must be marked with “TW” and include the cylinder, valve, and permanent attachments. If you want to know exactly how much propane is in your tank, weigh it and do the subtraction:
- Find the stamped TW number on the cylinder’s collar.
- Place the tank on a bathroom scale and record the total weight.
- Subtract the tare weight from the total weight.
The result tells you the actual pounds of propane inside.
When you’re ready to replace your cylinder, our tested roundup of the best 20 lb propane tanks compares top-rated models side by side.
Why the Distinction Matters
The difference between fuel weight and gross weight isn’t academic. It affects how long your grill runs, whether you’re getting fair value from an exchange program, and whether your tank is actually full when the gauge says otherwise.
Knowing your cylinder’s tare weight also helps you estimate fuel remaining mid-season. Weigh the tank, subtract the TW, and you’ll know exactly how many pounds of propane are left — no gauge needed. That simple calculation gives you real control over your fuel supply and prevents the dreaded mid-cookout flame-out.
A properly filled 20 lb tank contains 20 pounds of propane, roughly 4.6 to 4.8 gallons, and weighs about 37 to 40 pounds total. Exchange cylinders may hold less, so verify by weight before you assume you’re getting a full tank.
FAQs
How long does 20 lb of propane last on a grill?
High-heat grilling, cold outdoor temperatures, and frequent lid lifting all reduce burn time.
How can I tell if my propane tank is actually full?
Weigh the tank on a bathroom scale and subtract the tare weight stamped on the cylinder’s collar. A full 20 lb tank will show about 20 pounds of propane. You can also pour warm water down the side — the tank feels cool where propane remains and warm where it’s empty.
Why does my exchange tank feel lighter than my old one?
Compare the stamped tare weight and weigh the cylinder to verify the actual fuel amount.
References & Sources
- National Institute of Standards and Technology. “Liquified Petroleum Gas 20 Pound Cylinders.” Documents net weight labeling practices and exchange cylinder fill amounts.
- Eagle Propane Sales. “Cylinder Sizes & Specifications.” Provides dimensions, tare weights, and capacity specs for steel propane cylinders.
- AmeriGas. “Propane Tank Sizes.” Explains common residential cylinder sizes and their capacities.
