If You See a Swollen Gas Can, This Is What It Means

Updated: May 04, 2023

Ever noticed a plastic gas can that's either swollen outwards or collapsed inwards? Learn why this happens and what to do about it.

Gasoline is a great thing to have handy as a DIYer, since many useful machines burn it to run. Most folks who keep a lawn mower, weed-whacker, rototiller or car around understand the value of keeping a can or two of gas nearby for those times when driving to the nearest station isn’t practical. This is a simple enough concept, but what worries some people is the way plastic gas cans periodically change shape.

If you’re in the habit of keeping plastic cans of gas around, you’ve probably noticed how they sometimes swell beyond their normal size, and other times collapse inwards. Why do they change shape, and what, if anything, should you do about it?

Why Do Plastic Gas Cans Swell?

Plastic gas cans sometimes swell when vapor given off by the gas inside expands in warm temperatures. If this expanding vapor has no chance to vent due to a tightly closed gas can lid, the walls of the container will bulge outwards from the increased pressure. Gasoline is a fairly volatile substance, giving off considerable amounts of vapor, and this can lead to substantial swelling of a plastic gas can when temperatures rise.

Is It Normal and Safe for Gas Cans to Swell?

Woman Filling Up Gas Canister At Fuel StationWestEnd61/Getty Images

Most times swelling plastic gas cans are nothing to worry about. If your can starts developing cracks, it will, of course, need to be emptied and recycled, but otherwise, you needn’t worry about a bit of swelling. That said, if it makes you nervous there are two simple gasoline storage solutions that will help. You can either open the cap of your can for a few minutes to let the vapor escape before closing it again, or invest in a heavy-duty metal gas can that will never swell or collapse.

Why Do Gas Cans Collapse?

Like swelling, gas can collapse is caused by vapors coming off the liquid gasoline. When the air temperature drops, any vapor trapped inside a sealed plastic gas can contracts. This contraction reduces pressure in the can enough that the walls of the plastic can are pulled inwards, giving it a partially collapsed appearance. Once again, this issue can be swiftly solved by opening the lid and allowing the vapors to escape. Then set the gas can in the sun for a few minutes to warm up a little, and it should quickly be restored to its original shape.

Reader's Digest
Originally Published on Reader's Digest