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How Long Can Gas Sit in a Lawn Mower Before it Goes Bad?

By Jeff Mercer · Updated September 24, 2026 · 3 min read
How Long Can Gas Sit in a Lawn Mower Before it Goes Bad?
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Hey there, it’s your buddy Jeff here. I’ve had my share of mishaps with old gas in my lawn mower, and let me tell you, it’s not a good time.

So, I figured I’d write this article to help you avoid the same fate. If you’re wondering how long gas can sit in a lawn mower before it goes bad, well, read on and I’ll fill you in.

How Gas Goes Bad

First off, let’s talk about how gas goes bad. Gasoline is made up of a bunch of different chemicals, and over time, these chemicals start to break down and form gunk and contaminants.

This process is accelerated by heat, moisture, and air exposure, so if you’ve got a can of gas sitting in your garage that’s been open for a while, it’s likely to go bad faster than a sealed can of gas that’s been stored in a cool, dry place.

How Long Gas Can Sit Before it Goes Bad

So, how long can gas sit before it goes bad? Well, it depends on a few factors, including the quality of the gas, the storage conditions, and the age of the gas.

In general, though, most experts agree that gas starts to go bad after about three to six months. After that point, it’s likely to start forming gunk and contaminants that can cause problems for your engine.

What to Do with Old Gas

If you’ve got old gas in your tank and you’re not sure if it’s still good, there are a few things you can do:

  1. Drain the old gas out of the tank and refill it with fresh, high-quality gas.
  2. Add a fuel stabilizer to the old gas to help extend its shelf life.
  3. Dispose of the old gas properly (check with your local waste management authority for guidelines).

FAQ

How Long Can Gas Sit in a Lawn Mower Before it Needs to be Changed?

In general, it’s a good idea to change the gas in your lawn mower (or any other gas-powered equipment) after about three to six months.

After that point, the gas is likely to start going bad and forming gunk and contaminants that can cause problems for your engine.

Can You Use Old Gas in a Lawn Mower?

It’s generally not a good idea to use old gas in a lawn mower (or any other gas-powered equipment).

Old gas can cause all kinds of problems, including engine damage, poor performance, and difficulty starting.

If you’ve got old gas in your tank and you’re not sure if it’s still good, it’s best to drain it out of the tank and refill it with fresh, high-quality gas.

How Do You Store Gas for a Lawn Mower?

If you want to store gas for your lawn mower (or any other gas-powered equipment), here are a few tips:

  • Store the gas in a clean, approved container.
  • Keep the container in a cool, dry place.
  • Make sure the container is sealed tightly to prevent air from getting in.
  • Add a fuel stabilizer to the gas to help extend its shelf life.

Conclusion: Don’t Let Your Gas Go Stale

In conclusion, it’s important to pay attention to the age of your gas when using a lawn mower (or any other gas-powered equipment).

Gas starts to go bad after about three to six months, and using old gas can cause all kinds of problems, including engine damage, poor performance, and difficulty starting.

If you’ve got old gas in your tank and you’re not sure if it’s still good, it’s best to drain it out of the tank and refill it with fresh, high-quality gas. And if you need to store gas for a while, make sure to follow the tips above to help extend its shelf life.

So, remember, don’t be like me and end up stranded in the middle of your lawn with a dead mower and a whole lot of grass to cut.

Keep your gas fresh, and you’ll have a smooth, hassle-free mowing experience. Happy mowing!

Before you start the repairWe are not certified small-engine technicians. Before any repair, pull the spark plug wire and let the engine cool. Anything involving blades under a raised machine, fuel system leaks or transmission internals — if you're not sure, have a dealer or small-engine shop look at it.

Written by

Jeff Mercer

Jeff Mercer writes GardenTooly's answers to the questions people type into a search bar while standing next to a machine that won't behave: why the deck shakes, why the mower dies when it warms up, whether a bent blade shaft can be straightened, what the difference between two transmissions really means for you, and which fuel additive is worth the shelf space.

Each answer starts with the cause that is most likely and cheapest to check. Where it comes from the owner's manual, a service bulletin or what owners report, it says so, and anything involving blades, fuel, a jacked-up machine or a hydrostatic transmission gets a plain "take it to a shop" when that's the right call.

A note on our names: GardenTooly is written by a small team under pen names, one name for each job, and Jeff Mercer is the name on our mower and small-engine guides. We are not certified small-engine technicians, and we don't sell equipment.

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