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Ask the Expert

Posted: May 6, 2019

Lenny Adams

Q: I’ve noticed that after a delivery, my propane tank is never filled all the way. Why not?

A: The propane in your tank is a liquid, which changes to gas before it leaves the tank. But like all liquids, it will expand when its temperature rises. To make sure there’s room for propane to safely expand, we typically fill tanks to about 80%. So a 500-gallon tank that’s 80% full will safely hold about 400 gallons of propane.

Keep in mind the amount of propane in your tank doesn’t change when it expands and contracts, but its density does.

We call this the 80/20 rule, and it’s especially important in hot weather, and in aboveground tanks, which are not insulated from heat as an underground tank is. Propane gas expansion is also the reason why you should never paint your propane tank a dark color: Dark colors absorb more heat.