How many gallons of propane are in a 120-gallon tank?

If that sounds like a trick, it is: The tank has a 100-gallon capacity. And the same math holds for the five-gallon tank you hook up to the barbecue — it tops off at four gallons, max. So why aren’t propane tanks ever filled all the way up? That is the 80-percent question!

The 80-percent fill is a preventative safety measure against the fluctuations that happen inside a tank. Propane, like other gases, expands when it’s hot and contracts when it’s cool. That means a tank that is 80-percent full on a mild March day, might register as 85-percent (or higher) at the mid-July barbecue. It’s the same amount of propane, but it’s taking up more space. So the extra headroom is a cushion against the pressure that builds up in a tank when it’s hot out.

The short-fill has become a standard in the marketplace, as most grill masters know. Since 2002, upright tanks between four and 40 pounds are required to have an overfill protection device (OPD) — a simple device that automatically shuts off a fill when the tank is about four-fifths full. Larger propane tanks, though not fitted with OPDs, also follow the 80-percent rule.

Want to know the fill capacity of your propane tank? Multiply the tank’s total capacity by 0.8.

If you have more propane questions or need to find an affordable propane provider, check out our rates today at