Gas Shelf-Life & Stabilizer Calculator

Quick answer: there are two published answers and they do not agree. Briggs & Stratton says fuel “can begin to deteriorate in as little as 30 days” and tells you to buy gas in quantities you will use inside a month. AAA says gasoline generally lasts three to six months, and that ethanol blends — which is almost all US pump gas — lose combustibility in one to three months. A stabilizer added to fresh fuel stretches that: STA-BIL Storage claims up to 24 months, Sea Foam up to 2 years, and Briggs’ own treatment up to 3 years. Nothing rescues fuel that has already gone off.

How to use it

Type how long ago you filled the can, pick the fuel and say where it has been sitting. The calculator puts your can on a timeline next to both published windows, so you can see which side of each one you are on, and then tells you what to do — use it, use it first, smell it, or drain it. If you are treating a fresh can, the dose for every gallon you have is at the bottom, taken from the product’s own label.

The one thing it will never tell you is that a fuel is “safe”. That is not ours to say. What we can do is show you who published what, and let you decide with the manual in your hand.

Why the two answers are so far apart

They are answering different questions. The fuel companies and motoring clubs are talking about gasoline in a sealed container, going into a car with a sealed, pressurised fuel system and an injector that will forgive a lot. Briggs is talking about a carburettor with passages the width of a pin, sitting in a shed, in a machine with a vented tank. Gasoline loses its light, easily ignitable fractions first — the very part that makes a cold engine catch — and a small engine notices that long before a car does.

So the honest answer to “how long does gas last?” is: longer than 30 days for most purposes, shorter than six months for a mower, and it depends on which machine you are pouring it into. Any page that gives you one number has picked a side without telling you.

What ethanol actually does

E10 — up to 10% ethanol — is what comes out of nearly every pump in the US. Ethanol is hygroscopic: it pulls water out of the air. A vented can or tank breathes every time the temperature changes, and each breath brings a little moisture in. Take on enough and the ethanol and water separate out and sink, which puts a corrosive water-and-alcohol layer exactly where the fuel pick-up sits. That is the usual reason a machine that ran fine in October will not start in April, and it is why ethanol-free fuel, where you can buy it, is worth the premium for anything that sits.

E15 is a different matter entirely. Briggs & Stratton’s own fuel recommendation says: “Some fuel stations are now selling gasoline with up to 15% ethanol. This E15 product is not recommended or approved for use in small engines”, and that using unapproved fuel “will damage the engine components, which will not be covered under warranty”. At the pump it is often badged Unleaded 88, which reads like an upgrade and is not one. Check the label before the nozzle goes in the can.

Stabilizer: what it does, and what it does not

A fuel stabilizer slows oxidation and keeps gum and varnish from forming while fuel sits. Used on fresh fuel it works well, and the makers put numbers on it:

What it does not do is reverse anything. Every label says to add it to fresh fuel — STA-BIL’s directions are literally “Use STA-BIL only with fresh fuel” — so stabilizer poured into a can that already smells sour buys you nothing but a more expensive can of stale gas. Add it on the day you fill up, run the engine for a few minutes so the treated fuel reaches the carburettor, and keep the can nearly full, because the air in a half-empty can is what brings the water in.

HEET is not a stabilizer

This one catches people out, and there are a dozen articles on this site about it. HEET Gas-Line Antifreeze & Water Remover does exactly what the bottle says — it removes water from the fuel system and stops a fuel line freezing. That is a real job and it is worth doing. But it is not sold as a storage stabilizer, and a can treated with HEET and left over the winter is, as far as staleness goes, an untreated can. Use HEET for water; use a stabilizer for time.

How to tell by looking

Fresh gasoline is clear, thin, and smells sharp. As it ages it goes darker — towards the colour of weak tea — and the smell turns sour and varnishy rather than sharp. Water shows up as beads or a separate layer at the bottom of a clear container; if you can see a line, the fuel above it is not usable as it stands. A fuel that fails any of those three tests is not worth the afternoon you will spend cleaning the carburettor afterwards.

Getting rid of fuel you cannot use

Briggs & Stratton’s instruction is the one to follow: “If gasoline in the small engine has not been treated with a fuel stabilizer, it must be drained into an approved container. Run the engine until it stops from lack of fuel.” Do it outdoors, with the engine cold, into a container made for fuel. Small amounts of mildly stale gas can be diluted into a car’s tank — a gallon into a nearly full tank — but do that with judgement, not with fuel that has water in it or smells like varnish. Anything worse goes to a household hazardous-waste site; most counties run one free for residents. Never down a drain, never on the ground, and never into a fire.

What this does not cover

Diesel, kerosene and canned pre-mixed fuel all age differently, and mixed two-stroke fuel has an oil in it with its own life — mix what you will burn, and use the 2-cycle gas mix calculator so you are not left with a gallon of mix and nothing to put it in. If your machine will not start after a winter, the fuel is only half the story: the carburettor it sat in is the other half.