What is the ratio for 2-stroke fuel?

It is engine-specific and the manual is the authority — 50:1 is the most common ratio on modern chainsaws and trimmers, but 40:1 and 32:1 are specified on some engines, and older equipment sometimes calls for 25:1. At 50:1 that is 2.6 fl oz of two-stroke oil per US gallon of gasoline. Running too little oil seizes an engine; running too much fouls plugs and carbons the exhaust port. Never guess a ratio.

How much 2-stroke oil per gallon at 50:1?

2.6 fl oz of oil per US gallon of gasoline. For smaller quantities: 1.3 fl oz per half gallon, and 0.6 fl oz per quart. At 40:1 it is 3.2 fl oz per gallon, and at 32:1 it is 4 fl oz per gallon. Most two-stroke oil bottles are sold in these exact dose sizes precisely so you can empty one bottle into one or two gallons without measuring.

What gas should you use in a chainsaw?

Fresh unleaded of at least 89 octane where the manufacturer specifies it, with ethanol content no higher than 10 percent — most manufacturers prohibit E15 and above, and it will void the warranty. Ethanol-free gasoline is markedly better if you can find it. Never use E85, never use gasoline more than 30 days old, and never use fuel from a can that has been open for a season.

How long does mixed 2-stroke fuel last?

About 30 days in a sealed container without stabilizer, and up to about 90 days with a fuel stabilizer added at mixing. Ethanol-blended gasoline absorbs atmospheric water and phase-separates, and the two-stroke oil begins settling out at the same time. Canned pre-mixed fuel is stabilized and ethanol-free and stores for years unopened, which is why it is the sensible choice for equipment used a few times a season.

Why won't my chainsaw start after winter?

Stale fuel, in the great majority of cases. Ethanol-blended gasoline degrades within weeks and leaves varnish deposits in the carburetor jets, which is enough to stop the engine drawing fuel. Prevention is to store the saw with the tank empty and the carburetor run dry, or to run canned pre-mixed fuel year-round. Fixing it usually means draining, fresh fuel, a new plug and often a carburetor clean.

How to Mix 2-Stroke Fuel (Ratios, Method, Storage)

Written & researched by Priya Anand DIY & Tools Editor

Updated October 2026

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Mix two-stroke fuel at the ratio in your engine's manual — most modern chainsaws specify 50:1, which is 2.6 fl oz of two-stroke oil per US gallon of fresh gasoline. Pour about half the gasoline into an approved fuel can, add all of the oil, cap it and shake, then add the remaining gasoline and shake again. Use fuel with 10 percent ethanol or less, add stabilizer if it will sit more than a month, label the can with the ratio and date, and use it within 30 days. Ratios are not universal: 40:1 and 32:1 exist, and using the wrong one either seizes the engine or fouls it.

Ratio chart

Ratio	Oil per 1 US gallon	Oil per 1/2 gallon	Oil per quart	Typically specified for
50:1	2.6 fl oz	1.3 fl oz	0.6 fl oz	Most modern chainsaws, trimmers, blowers
40:1	3.2 fl oz	1.6 fl oz	0.8 fl oz	Some saws and older equipment
32:1	4.0 fl oz	2.0 fl oz	1.0 fl oz	Older two-strokes, some commercial units
25:1	5.1 fl oz	2.6 fl oz	1.3 fl oz	Vintage equipment; check the manual

Use the ratio your manual specifies, not the one you used on your last machine. Too little oil means insufficient lubrication of the crank and cylinder, and a seized engine is usually a scrapped engine. Too much oil fouls the spark plug, carbons the exhaust port and muffler screen, and makes the saw smoke heavily and run poorly. Neither error is cheap.

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What to buy

Two-stroke oil rated for air-cooled engines, meeting ISO-L-EGD or JASO FD. Do not use outboard two-stroke oil, which is formulated for water-cooled engines, and never use motor oil.
Fresh gasoline, 89 octane or as specified, with no more than 10 percent ethanol. Most manufacturers prohibit E15 and above and will not honor a warranty claim on fuel damage.
Ethanol-free gasoline if a station near you sells it. It stores far better and eliminates the main cause of carburetor trouble.
A fuel stabilizer, added at mixing, if the can will sit beyond a month.
An approved fuel container with a proper spout, and a funnel. Label it with the ratio and the date you mixed it.
Or skip all of it: canned pre-mixed two-stroke fuel is stabilized, ethanol-free, correctly proportioned and stores for years. It costs several times as much per gallon and is the right choice for equipment used a handful of times a season.

Mixing, step by step

Check the manual for the ratio. It is printed on the fuel cap or in the specifications section. Do not assume 50:1.
Work outdoors, away from ignition sources. No smoking, no running engines, no pilot lights. Gasoline vapor is heavier than air and travels along the ground.
Pour roughly half the gasoline into the can. Never mix in the saw's fuel tank — there is no way to blend it and the first thing the engine draws is neat gasoline or neat oil.
Add the full measure of oil. Use the dosing chamber on the bottle or a measuring cup. Eyeballing it is how engines get destroyed.
Cap and shake for 15 to 20 seconds. The oil must be genuinely in solution, not floating.
Add the remaining gasoline and shake again. The two-stage method disperses the oil far better than pouring everything in at once.
Add stabilizer now if the fuel will sit more than a month, and shake again.
Label the can with the ratio and the date. A can of unmarked mix is a can you will end up throwing away.
Shake before every fill. Oil settles slowly in a can left standing, and a shake takes two seconds.
Wipe up spills and let vapors clear before starting the engine.

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Why ethanol is behind most chainsaw starting problems

Read enough one-star reviews of gas chainsaws and one story dominates: the saw ran fine, sat for a season, and now will not start. The cause is fuel chemistry, not build quality.

Ethanol is hygroscopic — it draws water out of the air. In a vented fuel can or a saw tank, ethanol-blended gasoline gradually absorbs moisture, and past a certain point the mixture phase-separates: a layer of water and ethanol drops to the bottom while the gasoline floats above. The two-stroke oil separates alongside it. What is left in the carburetor when the light fractions evaporate is varnish, and varnish in a jet the diameter of a pin is enough to stop the engine drawing fuel entirely.

The timescale surprises people. This begins in weeks, not months, and it happens faster in hot sheds and in humid climates. It is the single most common reason a two-stroke saw fails, and it is entirely preventable.

Storing fuel and storing the saw

Situation	What to do
Using the saw weekly	Mix one gallon at a time, use within 30 days
Using it monthly	Mix with stabilizer, use within 90 days
Using it a few times a season	Buy canned pre-mixed fuel — cheaper than a carburetor rebuild
Storing over winter	Drain the tank, then run the engine until it stops to empty the carburetor
Fuel older than 90 days	Do not use it — dispose of it at a household hazardous waste site
Can left open or unlabeled	Assume it is contaminated and dispose of it

Running the carburetor dry is the step people skip and it is the one that matters most. Draining the tank leaves fuel sitting in the carburetor bowl and passages, which is exactly where varnish causes trouble. Start the engine after draining and let it run until it stalls.

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Mistakes that cost engines

Guessing the ratio. Lean mixes seize engines. Rich mixes foul plugs and carbon exhaust ports.
Mixing in the saw's tank. There is no way to blend it, and the engine draws unmixed fuel.
Using outboard or automotive oil. Wrong formulation for an air-cooled engine; it will not protect the cylinder at operating temperature.
Using E15 or E85. Prohibited by most manufacturers, damaging to fuel lines and seals, and a warranty void.
Using fuel from last season. This is the number one cause of a saw that will not start.
Storing the saw full of untreated mix. Guarantees a carburetor problem next spring.
Not shaking the can before filling. Oil settles, and the last fills from a standing can run lean.

If the saw is smoking heavily, an over-rich mix is one of the possible causes — the diagnosis is in why is my chainsaw smoking.

Safety

Mix and fuel outdoors, never in a closed garage, with no ignition source nearby. Let a hot engine cool for several minutes before refuelling — fuel spilled on a hot muffler ignites. Move at least 10 feet from the fueling spot before starting the saw, and never refuel with the engine running. Store fuel in an approved container, out of sunlight and away from living spaces and appliances with pilot lights, and dispose of old fuel through a household hazardous waste program rather than pouring it out.

The cutting itself carries the greater risk. Kickback is the mechanism behind most serious chainsaw injuries: contact between the upper quadrant of the bar nose and any solid object drives the saw backward and upward toward the operator's head and shoulder in a fraction of a second. Never cut with the nose, keep the tip in view, wrap your thumb fully under the front handle, stand slightly to the side of the cutting line, and keep the chain sharp and correctly tensioned. OSHA lists a hard hat, safety glasses, hearing protection, heavy gloves, cut-resistant chaps and ankle-covering boots as standard chainsaw kit — see best chainsaw chaps. Gas saws exceed NIOSH's 85-decibel eight-hour exposure limit immediately, so hearing protection is mandatory. Never cut from a ladder or above shoulder height, and hand leaning, split, hollow or power-line-adjacent trees to a certified arborist.

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How to Mix 2-Stroke Fuel