Gasoline is a blend of dozens of hydrocarbons, and the lightest of them start evaporating the moment the fuel leaves the pump. What remains oxidizes into gum and varnish, and the ethanol in most pump fuel pulls in moisture from the air. The result is fuel that will not start an engine, and a carburetor that needs cleaning. Knowing the timelines lets you plan storage so this does not happen.
Quick answer
Ordinary pump gasoline with ethanol stays reliable for about three to six months in a sealed, approved container, and as little as one to three months in a vented equipment tank. A fuel stabilizer added to fresh fuel extends storage to a year or more. Ethanol-free gasoline lasts longer. Diesel keeps six to twelve months, longer with treatment, and propane keeps indefinitely.
| Fuel | Untreated, sealed container | With stabilizer | Notes |
|---|---|---|---|
| Gasoline with ethanol (E10) | 3 to 6 months | 12 months or more | Absorbs water; the most common cause of starting trouble |
| Ethanol-free gasoline | 6 to 12 months | Up to 2 years with some products | Worth the premium for stored equipment |
| Pre-mixed small-engine fuel (canned) | 2 years or more sealed | Not needed | Ethanol-free, stabilized; opened cans last about 2 years |
| Diesel | 6 to 12 months | Longer with stabilizer and biocide | Keep cool, full and dry; water breeds microbial growth |
| Propane | Indefinite | Not needed | Cylinders need requalification after 12 years, then periodically |
In this guide
What happens to gasoline over time
- Evaporation. The volatile components that make cold starting easy leave first, especially in a vented tank or a half-empty can.
- Oxidation. Fuel reacts with oxygen to form gum and varnish that coat carburetor jets, needles and injectors.
- Phase separation. Ethanol absorbs water from the air. Once it holds more than it can carry, the ethanol and water drop to the bottom of the tank as a separate layer, leaving lower-octane gasoline on top and a corrosive water mix at the fuel pickup.
Heat and air speed all three up. A sealed metal or approved plastic container in a cool shed does far better than a half-full mower tank in a hot garage.
How fuel stabilizers help
Stabilizers are antioxidants with additives that slow oxidation and, in some products, help manage moisture. Added to fresh fuel at the dose on the bottle, most claim storage of 12 to 24 months. They cannot restore fuel that has already gone bad. Use them at the fuel can when you fill it, so everything that goes into your equipment is already treated, and run the engine a few minutes after adding stabilizer so treated fuel reaches the carburetor. See how to store a generator for the full routine.
See fuel stabilizers on Amazon
Signs that gas has gone bad
- A sour, varnish-like smell instead of the sharp smell of fresh fuel.
- Darker color, or a cloudy or layered appearance in a clear container.
- Hard starting, surging, stalling or loss of power in an engine that was fine before storage.
- Gum visible inside the carburetor bowl or on the fuel filter.
Slightly aged fuel can be diluted with fresh fuel and burned in a car, where the injection system and a large tank are forgiving. Fuel that smells bad, looks dark or has separated should be disposed of.
Storing gasoline safely
- Use approved containers, red for gasoline, yellow for diesel, and fill them to about 95 percent to leave room for expansion.
- Store them in a detached shed or garage, out of sunlight, away from any flame, pilot light, furnace or water heater, and never in living space.
- Date the containers and rotate the fuel through your car every few months.
- Keep only what you need. Most fire codes limit the gasoline a household may store, with 25 gallons a commonly cited ceiling, and homeowners’ insurance may have its own rules. Check your local code.
- Keep a class B fire extinguisher nearby.
Disposing of old fuel
Never pour fuel on the ground, down a drain or into the trash. Most communities have a household hazardous waste site or collection day that accepts gasoline, and some auto shops and recycling centers take it. Transport it in an approved container and tell them what it is.
Diesel and propane
Diesel does not evaporate like gasoline but it does oxidize, absorb water and support microbial growth in tanks that contain water. Keep stored diesel cool, full and dry, treat it with a stabilizer and a biocide for long storage, and drain water from tank bottoms. Propane does not degrade; its limits are the cylinder’s inspection date and its valves and seals. Our guide to dual-fuel generators explains why that makes propane popular for emergency equipment.
Frequently asked questions
Can I use gasoline that is a year old?
If it was stored sealed with stabilizer, probably. Untreated gasoline a year old is likely to cause starting trouble in a small engine. Smell it and look at it; if in doubt, mix a little into a car's tank rather than risk a carburetor.
Does gas go bad faster in a car or a lawn mower?
In the mower. A car's tank is sealed and its fuel is used quickly. A mower's tank is vented, small and often sits half-full for months, which is the worst case for evaporation and water absorption.
Is ethanol-free gas worth it for small engines?
For equipment that sits between uses, usually yes. It stores longer, does not absorb water the way ethanol fuel does, and is kinder to older carburetors and rubber parts. For engines used daily it matters less.
Does fuel stabilizer fix old gas?
No. It slows the breakdown of fresh fuel but cannot reverse oxidation or remove water. Old gas should be diluted into a larger tank if it is only slightly aged, or disposed of if it has gone bad.
