Stihl Chainsaw Oil Mix Calculator

Stihl's official ratio for its 2-stroke chainsaws, using genuine STIHL 2-stroke engine oil, is 50:1 — 50 parts petrol to 1 part oil. In practical terms, that is 100 ml of oil for every 5 litres of petrol, or about 20 ml of oil per litre. Always check your specific saw model's owner's manual, as some models or other oil brands may specify a different ratio.

At Stihl's standard 50:1 ratio, 5 litres of petrol needs 100 ml of 2-stroke engine oil, giving 5.1 litres of mixed fuel with the oil making up about 2% of the total. This is the exact example Stihl itself publishes in its official mixing guidance.

Running a 2-stroke engine leaner than its specified ratio (less oil than recommended) is the higher-risk direction. Inadequate lubrication can cause the piston to overheat and seize against the cylinder wall, and in severe cases can bend or snap the connecting rod or crankshaft — this is usually catastrophic and can mean a full engine rebuild or replacement. Never intentionally run leaner than your manufacturer's specified ratio to try to "save" oil.

A richer-than-specified mix (more oil than recommended) is not usually damaging in the short term, but causes symptoms like visible blue-grey smoke from the exhaust, an oily residue around the muffler, fouled spark plugs with a black, wet appearance, sluggish throttle response, and carbon buildup on the piston crown over time with repeated use.

Some other manufacturers, older 2-stroke engines, or specific oil formulations use ratios like 40:1, 32:1, or 25:1 instead of 50:1 — this calculator supports any custom ratio so you can match your own saw's requirements. Always follow your saw's owner's manual rather than assuming 50:1 applies universally; using a richer ratio than your manual specifies is not dangerous, but using a leaner one than specified can be.

Yes. Use a high-quality 2-stroke oil with at least a TC (or equivalent) classification, formulated for air-cooled 2-stroke engines like chainsaws — oil designed for other applications (such as 2-stroke outboard motor oil) may not provide the same protection. Genuine STIHL 2-stroke oil is specifically formulated for STIHL tools' 50:1 ratio.

Mixed 2-stroke fuel is generally recommended for use within about 30 days, since ethanol-blended petrol can separate and both the petrol and the oil can degrade over time, leading to starting problems, rough running, or engine damage from stale or separated fuel. Buying pre-mixed fuel (like Stihl MotoMix) or mixing only what you will use within a month avoids this.

Stihl recommends a minimum octane rating of 90 RON, and notes that the standard unleaded petrol widely available at UK filling stations (95, 97 or 98 RON) is suitable. E10 petrol (up to 10% ethanol) is generally suitable for STIHL tools, though ethanol-blended fuel should not be stored mixed for long periods.

Stihl's own guidance is to pour the oil into an approved mixing canister first, then add the petrol, seal the canister, and shake it vigorously by hand to combine the mixture thoroughly before use. Mixing in this order and shaking well helps ensure the oil disperses evenly rather than settling.

Yes — select "Custom" and enter your own saw's specified ratio. Most modern chainsaw brands specify a 50:1 or similar ratio, but always check your specific model's owner's manual rather than assuming another manufacturer's figure applies, since ratios and recommended oil types do vary between brands and models.

Pre-mixed fuel like Stihl MotoMix is already blended at 50:1 with high-quality, ethanol-free fuel and STIHL 2-stroke oil, removing any risk of mixing error and avoiding ethanol-related storage problems, at a higher cost per litre than mixing your own. For occasional or infrequent use, or for equipment left unused for long periods, pre-mixed fuel is often considered good practice; for regular heavy use, hand-mixing to the correct ratio is the normal and cost-effective approach.

Small 2-stroke air-cooled engines like chainsaws are more sensitive to fuel quality and ethanol content than most modern car engines, and have no onboard computer to compensate for a poor mix or fuel degradation, which is why manufacturers give specific octane and mix guidance rather than "any unleaded petrol will do."

A 2-stroke engine (like most chainsaws) needs oil mixed directly into the petrol because it has no separate oil sump or reservoir — the oil in the fuel is what lubricates the engine's internals as it burns. A 4-stroke engine has a separate oil reservoir like a car engine and runs on unmixed, straight petrol; putting mixed 2-stroke fuel in a 4-stroke engine (or vice versa) can cause serious damage.

Yes — enter any fuel amount in litres or US gallons, and the calculator scales the oil quantity proportionally to your chosen ratio. Many chainsaw users prefer to mix smaller batches (1-2 litres) more frequently rather than a large batch that might not be used within the recommended 30-day window.

Excess smoke is a common symptom of a too-rich fuel mix, but it can also be caused by an incorrectly adjusted carburettor, a dirty air filter, using the wrong type of 2-stroke oil, or normal behaviour on a cold start that should clear once the engine warms up. If smoke persists after confirming your mix ratio is correct, check the air filter and consider having the carburettor inspected.