Every fall I get the same phone call. Somebody fires up the generator they parked in the garage last spring, it sputters, coughs, and quits. Then they call me, convinced the carburetor is shot. Nine times out of ten, the carburetor is fine. The gasoline sitting in it went bad.
Gasoline feels like it should last forever. It’s a fuel, right? But the stuff that comes out of the pump today is a chemistry project that starts breaking down the moment it hits air. If you’re storing fuel for a backup generator, a mower, or a chainsaw, knowing its real shelf life is the difference between power during an outage and a dead machine when you need it most.
So let’s talk honestly about how long gasoline lasts, why it goes off, and what you can actually do about it.
Key takeaways
- Untreated pump gasoline stays reliable for about 3 to 6 months in a sealed container.
- Ethanol-blended fuel (most E10 at the pump) degrades faster because it pulls in water.
- A fuel stabilizer can stretch usable life to 1 to 3 years if you add it while the gas is still fresh.
- Store gas in a sealed, approved container, kept cool and full, away from your living space.
How long does gasoline last before it goes bad?
Here’s the short version. Regular gasoline in a properly sealed container holds up for roughly three to six months before you notice a real drop in performance. In a vented can or a half-empty tank, figure closer to one to three months.
Two things kill gas over time: oxidation and evaporation. When fuel meets oxygen, the lighter compounds that make an engine start easily begin to break down and form gum and varnish. You’ve seen the result if you’ve ever poured old gas and caught that sour, turpentine smell.
The lighter, more volatile parts also evaporate first. What’s left behind is heavier, harder to ignite, and gummier. That’s why a six-month-old can might still light off a lighter but won’t reliably start a cold engine.
Signs your stored gas has turned
You don’t need a lab. Pour a little into a clear glass jar and look.
- Color has darkened from clear-gold to brown or rust-orange
- A sour or varnish-like smell instead of that sharp gasoline bite
- Cloudiness or a separate water layer at the bottom
- Sticky residue on anything the fuel has touched
Any of those, and I’d keep it out of your generator. Bad gas doesn’t just fail to start an engine; it leaves deposits in the carburetor that cost you a repair bill down the road.
Why ethanol makes everything worse
Most gas at American pumps is E10, up to 10 percent ethanol. Ethanol is cheap and burns clean, but it has one nasty habit for anyone storing fuel: it loves water.
Ethanol is hygroscopic, which is a fancy way of saying it pulls moisture straight out of the humid air. Give it enough time and the fuel absorbs so much water that the ethanol and water separate out and sink to the bottom of the tank. Mechanics call it phase separation, and it’s ugly.
On a service call last winter I opened up a homeowner’s portable generator that hadn’t run in about eight months. The bottom third of the tank was a milky layer of water and ethanol. That machine was never going to start, and the carburetor needed a full cleaning. All from gas that looked fine when he poured it in.
Never store gasoline inside your home, in a basement, or near a water heater, furnace, or any pilot light. Gasoline vapors are heavier than air, travel along the floor, and can ignite from a spark across the room. Keep it in a detached garage or shed.
If you can find ethanol-free gas, some stations sell it, often labeled for boats and small engines, it stores noticeably better. It costs more per gallon, but for fuel you plan to sit on, it’s usually worth it. If you want to weigh fuel choices for a generator specifically, our breakdown of generator fuel types compared lays out the tradeoffs.
How stabilizers change the math
Fuel stabilizer is the single cheapest insurance you can buy for stored gas. A bottle runs about $10 and treats a lot of fuel, most brands cover 25 gallons or more.
Stabilizer works by slowing oxidation and helping keep water in suspension so it doesn’t separate out. Products like STA-BIL or Star Tron, added correctly, can push usable shelf life from a few months out to somewhere between one and three years.
The catch, and this is the mistake I see all the time: stabilizer only works on fresh gas. It’s a preservative, not a cure. Pour it into fuel that’s already six months old and gummy, and you’ve just made expensive bad gas. Add it the day you buy the fuel, while it’s still clean.
After you add stabilizer to a generator’s tank, run the engine for 10 to 15 minutes. That pulls the treated fuel all the way through the carburetor so the whole fuel system is protected, not just the tank.
Storage shelf life at a glance
Everything depends on the fuel type and how you keep it. Here’s roughly what to expect under decent storage conditions, cool, sealed, and out of direct sun.
| Fuel type | Untreated | With stabilizer |
|---|---|---|
| E10 (standard pump gas) | 3 months | Up to 1 year |
| Ethanol-free gasoline | 6 months | 1 to 2 years |
| Diesel | 6 to 12 months | Up to 1 year+ |
| Propane | Indefinite | Indefinite |
Notice the last two rows. This is exactly why a lot of my customers who want long-term backup end up moving away from gasoline entirely. Propane simply doesn’t go stale, the tank you buy today will work in a decade. If that appeals to you, it’s worth reading up on the pros and cons of propane generators and roughly how much propane a generator uses during an outage.
Diesel splits the difference and stores longer than gas, though it has its own quirks with algae and water. For larger homes or long outages, diesel generators for home backup are worth a look.
How to store gasoline so it actually lasts
Getting six good months out of a can isn’t hard. It just takes a few habits.
Use an approved container. Red plastic gas cans or metal safety cans made to specification are designed to seal against vapor and pressure. Milk jugs and random containers are both dangerous and leaky.
Keep the container full. Air in the headspace is what feeds oxidation and lets moisture in, so a full can ages slower than a half-empty one. If you’re storing several cans, fill them to the line.
Store it cool and stable. Heat speeds up every chemical reaction that ruins fuel. A shaded shed that stays under 80 degrees beats a metal-roofed garage that hits 120 in July. The U.S. Department of Energy has solid reference material on fuel properties if you want to understand the chemistry, and the NFPA’s guidance on gasoline safety is worth two minutes before you set up any storage.
Label and date every container. Write the fill date and whether you added stabilizer right on the can with a paint marker. Six months from now you won’t remember, I promise.
Rotate it before you need it
The best trick I can give you costs nothing: use your stored gas before it ages out, and refill. Don’t let it sit until it’s a science experiment.
Pour last season’s stabilized fuel into your car’s tank a few gallons at a time, a healthy vehicle handles slightly older gas without complaint, then refill your cans fresh. I tell folks to set a reminder for spring and fall, same weekend they change the smoke detector batteries. That way your backup fuel is never more than about six months old, and your generator starts on the first pull when the lights go out.
While you’re at it, that seasonal check is a good time to run through your generator maintenance checklist and, if you’re still shopping, our beginner’s guide to portable generators. Fresh fuel and a maintained machine are the whole game. Get those two right and you’ll never be the person calling me in a panic when the power drops.
