Chainsaw Gas Oil Ratio: What Fuel Mix Does a Chainsaw Use?

This is the mistake that ends the most saws. A chainsaw has two separate reservoirs: the fuel tank, which takes the gas and 2-stroke oil premix, and a bar-and-chain oil tank, which takes a thick tacky oil that lubricates the chain as it runs over the bar. They are usually side by side under similar-looking caps.

Bar oil must never go in the fuel, and 2-stroke premix oil must never go in the bar oiler. Bar oil is not formulated to burn and will foul the engine quickly; premix oil is far too thin to stay on a moving chain. If the caps are unmarked, the fuel cap is the one closest to the starter and carburettor side.

Modern saws are almost universally 50:1. The exceptions worth knowing are older saws, which ran 40:1 or 32:1, and a small number of professional saws sold in markets where the manufacturer specified its own oil at a different rate. The decal on the fuel cap or tank is the authority; the manual is second best.

Where a saw has no decal and no paperwork, mixing 40:1 is the safer guess than 50:1. The extra oil costs you some smoke and a dirtier spark-arrestor screen, while guessing lean costs you a piston.

Brand-specific searches for chainsaw mix mostly land on the same answer. Current Stihl saws from the MS 170 up to the professional MS 500i, Husqvarna saws from the 120 Mark II to the 572 XP, and Echo saws from the CS-310 to the CS-590 Timber Wolf all specify 50:1, which is 2.56 oz of oil per US gallon or 20 ml per litre.

Where older saws differ, it is usually the oil that changed rather than the engine. Echo printed 32:1 in its manuals until the late 1990s and now says any older Echo engine can run 50:1 on Echo-branded oil. Older Stihl and Husqvarna manuals often gave a richer ratio for other brands of oil than for their own bottle, which is why 40:1 and 33:1 figures still turn up in forum answers about the same saw. With a current JASO FD or ISO-L-EGD oil, 50:1 is what all three makers ask for.

The brand oils are compared one by one in the Stihl, Husqvarna and Echo guides, and the same can of 50:1 runs the trimmer and leaf blower from those brands too.

The ratio belongs to the maker and the oil, not to the size of the saw. These are the figures the manufacturers publish today:

• Stihl: 50:1 for every gasoline saw, 2-MIX and 4-MIX alike, which Stihl UK writes as 5 litres of petrol to 100 ml of oil.
• Husqvarna: 50:1, which Husqvarna writes as 2% oil, for the whole saw range from the 120 Mark II to the 572 XP.
• Echo: 50:1 across the current range, from the CS-271T to the CS-620P. Older Echo manuals printed richer ratios, and Echo now approves 50:1 for those engines with Echo-branded oil.
• RedMax (Husqvarna group): 50:1, written as 2% in its current operator's manuals.
• Poulan, Craftsman, Homelite and other older or budget saws: 40:1 decals (3.2 oz per gallon) are common. Read the cap; these brands changed specs by model and year and publish few manuals online.

Every row above is measured with the maker's own oil or one of the same rating (JASO FD or ISO-L-EGD). The 2-stroke mix ratio statistics page counts how many current models sit at each ratio across saws, trimmers and blowers.

The Stihl chainsaw fuel ratio is 50:1 for every model, because Stihl sets it brand-wide: its US mixing guide says all Stihl gasoline-powered equipment uses a 50:1 gas and oil mixture. That covers the MS 172 and MS 180 homeowner saws (31.8 cc), the MS 250 (45.4 cc), the MS 271 Farm Boss (50.2 cc) and MS 291, and the professional MS 362, MS 462 and MS 500i (79.2 cc). One 2.6 oz bottle of Stihl oil into one US gallon of gas is the whole recipe; in metric it is 20 ml per litre, or 100 ml in a 5 litre can.

Two Stihl rules matter as much as the ratio. Stihl's fuel FAQ allows up to 10% ethanol (E10) and says no to E15, and it recommends not using mix older than 60 days, preferably 30, even with a stabilizer. Stihl MotoMix, its canned premix, is the exception: no ethanol, a storage life of up to two years, and Stihl says it can stay in the saw over the winter.

The Stihl chainsaw gas mix guide lists every current MS model with its displacement and the amounts for each can size, and the premixed fuel comparison sets MotoMix against TruFuel and mixing your own.

Stihl UK's advice is not to store fuel you mixed yourself for more than 30 days, because ethanol in pump gas pulls in moisture and the oil and gas begin to separate. Husqvarna's manuals say not to mix more than one month's fuel at a time, and Echo's say 30 days, or 90 with a stabilizer. A saw that cuts firewood twice a year is better served by a 1 gallon can than a 5 gallon one. Before the saw is put away for the season, winterizing it means running the tank dry rather than leaving that mix in the carburettor.

If the saw has been filled with the wrong thing, do not start it to see what happens. Drain the tank into an approved can, refill with correct 50:1 mix, and pull the plug if it will not fire. A tank of straight gas that has already been run for a few minutes needs a compression check or a look at the piston through the exhaust port before the saw goes back to work.

• Bogging or four-stroking at full throttle, with heavy smoke, usually means too much oil or stale fuel rather than a carburettor fault.
• Gradual power loss over a tank or two, then a saw that stops and will not restart, is the classic lean-on-oil seizure. Pull the muffler and look at the exhaust side of the piston: vertical scoring is the confirmation.
• A saw that runs but will not hold idle after sitting all winter is usually stale premix, not the ratio. Two-stroke mix begins to degrade within about 30 days unless it is stabilised.
• A spark-arrestor screen that clogs every few hours points at a mix that is too rich or an oil that is not a proper low-ash 2-stroke oil.

Use fresh 89 octane or higher, and ethanol-free fuel if you can get it. Ethanol attracts water and separates in storage, which is the leading cause of small-engine carburettor trouble. Mix only as much as you will burn in a month, label the can with the ratio and the date, and store it out of sunlight.

Before a saw goes into storage for the season, either run the tank dry or fill it with stabilised mix. Leaving a half-empty tank of untreated premix over the winter is what turns a working saw into a carburettor rebuild.