Why Is My Propane Tank Never 100% Full? It’s the 80% Question

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.