Storage is a turnover question
The instinct with fuel is to buy in the largest container available and refill rarely. That is right for anything that keeps and wrong for gasoline, because the constraint is not how much you can hold but how long the last gallon in the can has been sitting before it gets used. Two five gallon cans for a household that burns a gallon a week means fuel sitting for up to ten weeks. Two one gallon cans for the same household means fuel sitting for two.
The arithmetic is simple and worth doing once. Add up the machines, multiply gallons per hour by hours per week for each, and you have a weekly burn. Divide the fuel you keep on hand by that weekly burn and you have the age of the oldest fuel in the system. That single number tells you whether stabiliser is a precaution or a requirement, and whether the can sizes on your shelf are the right ones.
What actually goes wrong with stored fuel
Two separate things, and people conflate them. The first is that the light, volatile fractions in gasoline evaporate out over time, particularly from a container that breathes with temperature swings. What is left is harder to ignite, which shows up as an engine that cranks and cranks and then runs poorly, and as varnish deposits in a carburettor.
The second is water. Ethanol-blended gasoline attracts and holds moisture from the air, and when it holds more than it can carry the mixture separates, leaving a layer at the bottom of the can or the tank that is water and alcohol rather than fuel. Small engines suffer from this more than cars do, and the reason is not the fuel system design but the duty cycle: a car empties its tank every week or two and refills with fresh fuel, while a leaf blower sits for eleven months with a part-full tank in an unheated shed going through a temperature swing every day.
How long any of that takes is not a single number and anyone quoting one has left out the variables. It depends on the blend, on whether the container is sealed and how full it is, on the storage temperature and how much it swings, and on how much water is available in the air. That is why the usable life on this page is an input with a range rather than a figure supplied for you: your own experience with your own storage is better data than a general claim.
Stabiliser, honestly
Stabilisers work on the oxidation and gum-forming side of the problem, and they extend the window. They do not restore fuel that has already gone off, they do not remove water, and the multipliers on the labels vary enormously between products. The dose also varies by roughly a factor of ten across the market, so the number on the bottle in your hand is the only one worth using, which is why the calculator asks for it rather than assuming.
The one thing that is not in dispute is when to add it. Stabiliser goes into fresh fuel, ideally at the pump or straight into the can, because it slows a process rather than reversing one. Adding it to fuel that has already been sitting for two months does very little. If you treat everything as it comes in, storage becomes a much duller problem.
| Situation | What usually helps |
|---|---|
| Fuel turns over in a couple of weeks | Nothing. Storage volume is not your problem. |
| Fuel sits a month or two in season | Treat at fill, and buy smaller cans rather than more. |
| Machine used a few times a year | Treat, and run the machine after each fill so treated fuel reaches the carburettor. |
| Storing over winter | Either run the machine dry or fill and treat, following the manual. Half a tank is the worst option. |
End of season, and the safety part
The predictable failure is arriving at the last job of the year with a freshly filled five gallon can. Stopping the restocking two or three weeks before the season ends and running the cans down costs nothing and removes the entire problem. Whatever is left over should be either treated and stored deliberately, or used up in something that runs on straight gasoline where the consequences of slightly stale fuel are smaller.
Gasoline handling deserves one plain paragraph. Vapour is heavier than air, spreads along the ground and can find an ignition source at a distance from the can. Store fuel in containers made for it, keep them closed, keep them out of living space and away from water heaters, furnaces, pilot lights and anything that sparks. Never fuel a hot engine, because a muffler is far above the ignition temperature of what comes out of a tank. Fill containers on the ground, not in a truck bed or on a tailgate.
For the machines drinking all this, the run hours you entered here also drive the service interval planner. Mixed fuel gets its ratio from the two stroke fuel mix calculator. If a generator is part of the picture, its own consumption is modelled in the generator run time and fuel calculator and its sizing in the generator sizing calculator. And if you are burning fuel for the vehicle side as well, the fuel cost calculator handles that half.
Questions people ask
How long does gasoline last in a can?
There is no single honest answer, which is why this calculator makes it an input. What is stored, how it is stored and where all matter: the blend, whether the container seals or breathes, how full it is, the storage temperature and how much that temperature swings through the day. Fuel in a sealed can in a cool basement behaves very differently from the same fuel in a part-full can in a metal shed in July. Rather than adopting a number from the internet, use what you have observed with your own equipment and your own storage, set it as a range, and let the turnover figure tell you whether your storage volume fits inside it.
Why does ethanol matter more for small engines than for cars?
Mostly duty cycle rather than fuel system design. Ethanol-blended gasoline draws moisture out of the air and holds it in solution, and past a certain point the mixture separates into a fuel layer and a water-alcohol layer. A car burns through a tank every week or two and constantly refreshes it, so the fuel never sits long enough for much to happen. A trimmer sits for eleven months with a part-full tank going through a daily temperature cycle in an unheated shed, which is close to the worst case. The other factor is passage size: a small carburettor has jets fine enough that deposits which would be invisible in a car engine will stop it running.
Should I store equipment with a full tank or an empty one?
Follow the manual for the machine, because manufacturers differ and both approaches are defensible. The argument for full and treated is that a full tank has less air in it, and less air means less moisture and less room for condensation. The argument for empty is that fuel which is not there cannot go off or gum up a carburettor, and running the machine until it stops on its own clears the bowl as well as the tank. The one option nobody recommends is half a tank left untreated, which combines the drawbacks of both. Whichever you choose, if you treat the fuel, run the engine afterwards so treated fuel actually reaches the carburettor rather than sitting in the tank.
How much stabiliser do I add per gallon?
Read the label of the product you have, because doses across the market differ by roughly a factor of ten and using one product at another product concentration is either a waste or ineffective. Once you know the dose, this calculator turns it into ounces per can, ounces for the season and how many gallons a bottle will treat, which is the practical question when you are standing in front of a shelf. The other half of the answer is timing: stabiliser slows a process rather than reversing one, so it goes into fresh fuel at the fill, not into a can that has been sitting since spring.
Is it worth keeping a separate can of fuel for emergencies?
For storm cleanup or a generator, yes, and treating it as a separate rotating stock rather than padding your working supply is what makes it work. A dedicated can that gets treated when filled and then deliberately poured into a mower once or twice a year and refilled stays usable indefinitely, and it does not distort the turnover of the fuel you use week to week. Padding the working reserve instead means every gallon in the system sits longer, which is the exact problem you were trying to avoid. Whatever the volume, it belongs in approved containers stored away from living space and away from any ignition source.