The correct chainsaw fuel mix ratio is the petrol-to-two-stroke-oil ratio specified for your exact saw and oil; do not assume every saw takes 50:1. If your instructions specify 50:1, add 20 ml of the specified oil to each litre of petrol, or 100 ml to 5 litres.
This guide is for preparing a fresh batch for a petrol chainsaw whose manual requires premixed fuel. It does not identify an unknown liquid or diagnose engine damage.
Confirm what the two numbers describe
Write the ingredients beside the ratio before measuring anything. 50:1 petrol:oil means fifty volume units of petrol for each one of oil. STIHL sometimes writes the same relationship as 1:50 oil:petrol. The order of the ingredient names matters more than the direction of the colon.
Find the fuel section for the exact model and check the oil specification alongside the ratio. An oil bottle and a general brand page are useful evidence, but neither should silently replace the saw’s instructions.
There is a practical reason for this check: Husqvarna’s general mixing guide describes 50:1, while its separate ratio-support page distinguishes between its original oil and other oils. That does not establish a different ratio for your unidentified saw. If the manual, oil label and support advice disagree, ask the manufacturer or servicing dealer to resolve the combination before mixing. Give them the model and exact oil product, not just the brand of saw.
Calculate from the petrol you actually measured
Use matching units on both sides of the calculation. For a specified ratio of R parts petrol to 1 part oil:
Oil in millilitres = petrol in litres × 1,000 ÷ R
The formula converts litres to millilitres before dividing. It calculates how much oil to add to a measured quantity of petrol; it does not tell you which ratio your engine requires.
Here is a worked example, conditional on a confirmed 50:1 instruction:
- You measure 2 litres of petrol for a new batch.
- Convert that to 2,000 ml.
- Divide by the specified petrol-to-oil number: 2,000 ÷ 50 = 40 ml of oil.
- Check backwards before pouring: 40 × 50 = 2,000 ml of petrol.
STIHL’s published examples give 20 ml for one litre and 100 ml for five litres. The two-litre example above follows the same arithmetic. These are preparation examples, not reports of testing or a claim that 50:1 suits your saw.
The reverse check is useful because it asks a different question: “Would this oil quantity match the petrol I measured?” If it gives a different petrol quantity, resolve the discrepancy before combining the ingredients. Write the units with every number; an unlabelled “40” is difficult to audit later.
Catch three mistakes before pouring
Using the can’s capacity as the petrol quantity. A can marked 5 L does not establish that you put five litres into it. Calculate from the measured petrol going into this batch. An unknown remainder prevents a clean calculation, even when the new addition is measured accurately.
Treating the final mixture as the starting petrol volume. In the five-litre example, the instruction is five litres of petrol plus 100 ml of oil. Do not add oil and then top up to five litres of total mixture while calling that the same recipe. If a purpose-made mixing bottle uses fill lines, follow that bottle’s documented sequence and the correct ratio scale.
Losing the unit during calculation. This article uses litres and millilitres throughout. If your manual or measuring equipment uses gallons and fluid ounces, use its matching chart and stated unit system rather than inserting a gallon value into the litre formula. Do not assume an unqualified gallon chart matches your equipment.
Choose a measure that lets you read the required oil quantity clearly. Husqvarna cautions that measurement errors have a large effect on small batches. If the required amount falls between marks you cannot reliably read, obtain a suitable measure before proceeding.
Mix using the applicable manufacturer’s sequence
Use a clean container approved for petrol and leave the saw’s tank out of the mixing process. Husqvarna’s published method adds part of the petrol, then the full oil quantity, mixes, adds the remaining petrol and mixes again. STIHL describes an oil-first sequence. Follow the instructions applicable to your saw, oil and container instead of combining fragments of both methods.
Record the batch date, petrol quantity, oil product and measured oil quantity on a label. Include the confirmed petrol:oil ratio and the intended saw model. This is a batch record, not proof of compatibility with every other tool beside it.
Husqvarna advises preparing roughly a month’s supply and mixing again before refuelling. Use the applicable fuel instructions for storage limits; a commercial premix may have its own label requirements. If choosing premixed fuel, verify its stated ratio and suitability for the saw before use. For putting the machine away, use the separate chainsaw storage checklist.
An unknown batch is a stop decision
If you cannot establish what went into the can, the calculation above cannot recover the missing information. Adding a guessed amount of oil or petrol creates another unverified batch. Set that container aside from ready-to-use fuel and arrange handling or disposal through the appropriate local guidance.
STIHL says not to operate a saw containing an incorrect mix, even briefly. If wrong or unidentified fuel is already in the tank, keep the engine off and follow the model’s draining and refilling instructions, or have its servicing dealer handle it. Do not start the saw to “see whether it is close enough.”
If the saw was already run on suspect fuel, tell the dealer what was used and what happened. A later correct refill is not evidence that no damage occurred. Persistent stalling is a separate task covered by the starts-then-dies diagnostic guide.
Finish the fuel job before starting the saw
Correct arithmetic does not remove the fire precautions. Cornell’s chainsaw safety guidance says never to refuel a running or hot saw, prohibits smoking during fueling, and calls for dispensing fuel at least 10 feet from ignition sources. It also separates starting from the fueling area by at least 10 feet.
Use the manual’s cap-handling and filling procedure, prevent spills, and secure the containers afterward. Resolve spilled fuel before any start; STIHL also calls for changing fuel-contaminated clothing. Then perform the saw’s normal pre-use checks and prescribed starting procedure in the appropriate separate location. Keep the batch label readable for the next refill.
