2-Stroke Mix Calculation: Oil-to-Petrol Ratio 2026

What the 2-Stroke Mix Is and Why the Ratio Matters

Two-stroke engines — fitted on brushcutters, chainsaws, vintage scooters, motocross bikes, blowers and many other tools — don't have a separate lubrication circuit like four-stroke engines do. The oil needed to lubricate the cylinder, piston, bearings and crankshaft is mixed directly with the petrol in the tank. If the proportion is wrong, the consequences can range from a simple increase in exhaust smoke to a complete engine seizure. Calculating the mix correctly means protecting the engine, ensuring optimal performance and extending the working life of the tool or vehicle. In this guide we will go through everything you need to know: from standard ratios to ready-to-use tables, from the most common mistakes to practical tips for making a perfect mix every time. If you want an instant result, you can use our free online fuel-mix calculator, which gives you the exact amount of oil in seconds.

How a 2-Stroke Engine Works

To understand why the mix is so important, a brief overview of how a two-stroke engine works helps. Unlike a four-stroke engine, which completes the thermodynamic cycle in four piston strokes (intake, compression, power, exhaust), a 2-stroke engine completes the entire cycle in just two strokes — that is, in a single full turn of the crankshaft. On the upstroke, the piston compresses the air-petrol-oil mixture in the combustion chamber while at the same time creating a vacuum in the crankcase that draws in fresh mixture. On the downstroke, after combustion, the exhaust gases are expelled and the fresh mixture from the crankcase is transferred into the cylinder through the transfer ports. The oil in the mixture has three key functions: it lubricates the sliding surfaces between piston and cylinder, protects the crankshaft bearings, and helps seal the gap between the piston rings and the cylinder bore. Once burned together with the petrol, the oil is expelled with the exhaust gases: that is why 2-stroke engines produce more smoke than 4-strokes.

Standard Mix Ratios

The mix ratio expresses the proportion between petrol and oil. It is given as "X:1", where X represents the parts of petrol per part of oil.

50:1 ratio (2%)

It is the most widely used ratio today for most modern garden tools: brushcutters, chainsaws, blowers, hedge trimmers, tillers. It corresponds to 20 ml of oil per litre of petrol. Manufacturers like Stihl, Husqvarna, Echo, Makita and many others generally recommend this ratio when using a good-quality synthetic or semi-synthetic oil.

40:1 ratio (2.5%)

Used by some American and Japanese manufacturers, or when using a mid-range mineral oil. It corresponds to 25 ml of oil per litre of petrol. It is a good compromise for engines that work at medium-to-high revs for extended periods.

33:1 ratio (3%)

The recommended ratio for older engines, low-cost tools or when using traditional mineral oil. About 30 ml of oil per litre of petrol are used. It provides more protection at the cost of a bit more exhaust smoke and slightly more carbon build-up.

25:1 ratio (4%)

Typical of vintage scooters and 2-stroke bikes, older outboards and some industrial engines. 40 ml of oil per litre are used. The generous lubrication makes up for the lower quality of the oils available in the past and the wider mechanical clearances of vintage engines.

20:1 ratio (5%)

Rarely used today, it was the standard for engines from the 1960s and 1970s and for some very old outboards. It corresponds to 50 ml of oil per litre. The excess oil causes heavy smoke, carbon deposits and spark-plug fouling, but effectively protects engines with looser tolerances.

Complete Table: Oil Quantity per Litre of Petrol

Here is the ready-to-use table with the oil quantities required for the most common ratios. Values are expressed in millilitres (ml).

Petrol (litres)	50:1 (2%)	40:1 (2.5%)	33:1 (3%)	25:1 (4%)	20:1 (5%)
1	20 ml	25 ml	30 ml	40 ml	50 ml
2	40 ml	50 ml	60 ml	80 ml	100 ml
3	60 ml	75 ml	90 ml	120 ml	150 ml
4	80 ml	100 ml	120 ml	160 ml	200 ml
5	100 ml	125 ml	150 ml	200 ml	250 ml
10	200 ml	250 ml	300 ml	400 ml	500 ml
15	300 ml	375 ml	450 ml	600 ml	750 ml
20	400 ml	500 ml	600 ml	800 ml	1000 ml

For custom calculations with any quantity of petrol and ratio, use our online 2-stroke mix calculator.

The Formula for Working Out the Mix

The formula for working out the oil quantity is very simple: Oil (ml) = Petrol (ml) ÷ Ratio For example, to make 5 litres of mix at 2% (50:1 ratio): Oil = 5,000 ml ÷ 50 = 100 ml of oil For a 25:1 ratio with 3 litres of petrol: Oil = 3,000 ml ÷ 25 = 120 ml of oil If you instead know the desired oil percentage, the formula becomes: Oil (ml) = Petrol (ml) × Percentage ÷ 100 For example, for 3% with 5 litres: Oil = 5,000 × 3 ÷ 100 = 150 ml

How to Prepare the Mix: the Right Step-by-Step Procedure

Preparing the mix correctly doesn't just mean pouring in the right amounts: it means following a procedure that ensures a homogeneous, stable emulsion.

1. Choose the right container

Always use a fuel-rated jerrycan, preferably antistatic plastic with a pouring spout. Mix-dedicated cans are often green or carry a specific label to set them apart from pure petrol. Never use PET drink bottles: they are not resistant to hydrocarbons and can degrade, releasing particles into the fuel.

2. Measure precisely

For oil, use the graduated measuring cap supplied with the bottle or a lab-grade graduated cylinder. For small quantities (20–50 ml), a needle-free kitchen syringe is also very precise. Don't eyeball it: the error can be significant on small amounts.

3. Pour the oil in first, then the petrol

This is a fundamental step that many get wrong. By pouring the oil first into the empty can and then adding the petrol, the flow of fuel will mix the two liquids naturally. If instead you pour the oil into petrol that is already there, the oil tends to float and mixes much more poorly.

4. Shake well

After closing the can, shake it vigorously for at least 30 seconds. Repeat the operation before each use, especially if the mix has been sitting for a few days.

5. Don't store it for too long

The mix has a limited shelf life. After about 30 days the petrol starts to degrade (especially E10 ethanol-blended petrol), and the oil can partially separate. The ideal is to make only the quantity you expect to use within 2–4 weeks. If you have to store it longer, use a fuel stabiliser.

Synthetic, Semi-Synthetic or Mineral Oil: Which to Choose

The choice of oil affects not only engine performance, but also the mix ratio you should use.

Mineral oil

The most traditional and cheapest type, derived directly from crude-oil refining. It has good lubricating properties but produces more carbon residue and smoke. With mineral oil, richer ratios (25:1 or 33:1) are advisable. It is suited to older engines and budget tools used occasionally.

Semi-synthetic oil

A compromise between mineral and synthetic, it offers better anti-wear properties and burns cleaner than mineral oil. It allows intermediate ratios (33:1 or 40:1) to be used in most engines. It is the ideal choice for regular but non-professional use.

Synthetic oil

Chemically engineered to have uniform molecules and optimal properties, synthetic oil offers the highest level of protection, burns almost completely leaving very few deposits, and keeps its properties even at the highest temperatures. It allows lean ratios like 50:1 to be used while still ensuring excellent protection. It is the recommended choice for professional tools and for those looking for maximum performance.

Car engine oil in a 2-stroke: never!

A very common mistake is using car engine oil (such as 10W-40) in 2-stroke engines. 4-stroke engine oil is not designed to burn together with the fuel and leaves heavy deposits that clog the combustion chamber, exhaust port and muffler. It also contains additives that can damage the seals and specific materials of 2-stroke engines. Always and only use oil specifically formulated for 2-stroke engines, identifiable by the TC, TC-W (for marine use), JASO FC or JASO FD classifications.

Common Mistakes and How to Avoid Them

Too little oil (lean mix)

This is the most serious risk. A mix that is too lean causes insufficient lubrication, leading to overheating, scoring on the cylinder and, in the worst cases, the dreaded seizure: the piston locks in the cylinder, literally welding itself to the walls. Warning signs are a rise in operating temperature, an unusual metallic sound and a loss of power. If you notice these signs, switch the engine off immediately and check the mix.

Too much oil (rich mix)

Less dangerous than running lean, but still to be avoided. Excess oil causes heavy, dark exhaust smoke, spark-plug fouling (the plug gets coated with blackish deposits and stops working properly), carbon build-up in the combustion chamber and muffler, and a noticeable loss of power. Over time, deposits can clog the muffler and exhaust ports, dramatically reducing performance.

Using stale petrol

Modern petrol, especially ethanol-blended (E10, E15), degrades quickly. After 30–60 days it can turn acidic, form gummy deposits and cause starting and running problems. Always use fresh petrol and, if the tool is left idle for long periods, drain the tank or add a stabiliser.

Not shaking the mix

Even the best mix tends to separate over time. The oil, having a different density from petrol, can stratify. Before refilling, always shake the can for at least 15–20 seconds.

Mixes for Specific Applications

Chainsaws and brushcutters

Most modern manufacturers (Stihl, Husqvarna, Echo) recommend a 50:1 ratio with brand-specific synthetic oil. Stihl produces its HP Ultra (50:1), Husqvarna its XP Power 2T (50:1). In the absence of the original oil, use a JASO FD synthetic oil at 50:1 or a good semi-synthetic at 40:1.

2-stroke scooters (Vespa, Piaggio, Aprilia)

Older scooters with manual mixing (no separate oil pump) generally require a ratio between 2% and 3% (50:1 or 33:1), depending on the model and year. Models with an automatic oil pump (autolube) handle the mixing on their own and only require topping up the dedicated oil reservoir.

Motocross and enduro bikes

Competition 2-stroke bikes generally require a 40:1 or even 32:1 ratio, since they run at very high revs with significant thermal loads. Always check the bike's manual and use specific high-performance oils.

Outboard engines

For marine use, a specific TC-W3 oil (Two-Cycle Water-cooled) is used, formulated to resist humidity and contact with water. Ratios range from 50:1 for modern outboards to 25:1 for older models.

Petrol: Which Type to Use

For 2-stroke engines, unleaded petrol with the lowest possible ethanol content is generally recommended. Ethanol, present in E10 petrol (up to 10%), is hygroscopic — it absorbs moisture from the air — and can cause corrosion of seals and rubber components in older engines. If available, alkylate petrol (such as Aspen 2T or Husqvarna XP Power) is the ideal choice for small engines: it is virtually free of ethanol, benzene and aromatics, has a much longer shelf life (up to 3–5 years) and dramatically reduces harmful emissions. It costs more, but for low-consumption tools it is an investment that pays itself back in reliability and engine longevity. The recommended octane rating is generally 95 (SP95). 98-octane petrol does not offer significant advantages in most 2-stroke engines and costs more.

Frequently Asked Questions on the 2-Stroke Mix

Can I use leftover mix from last year?

No, it is strongly discouraged. Petrol degrades over time, especially the ethanol-blended kind. After 2–3 months the mix can cause starting issues, irregular running and damage to internal components. Dispose of it at a hazardous-waste collection point and make a fresh mix.

What happens if I get the ratio wrong?

If you put in too much oil, the engine will smoke a lot but will not suffer serious damage: burn through that mix and correct the dosage next time. If you put in too little oil, the risk is serious: add oil directly to the tank, shake, and run the engine at low revs for a few minutes before pushing it.

Is the chainsaw mix also fine for the brushcutter?

Yes, if the ratio is the same. A 50:1 mix with synthetic oil is universal for almost all modern garden tools. Always check each tool's manual, but you can generally make a single mix for all of them.

How do I know the right ratio for my engine?

Check the user manual of the tool or vehicle. If you no longer have it, look up the model on the manufacturer's website. In the absence of specific information, a 40:1 ratio with semi-synthetic oil is a cautious, safe choice for most 2-stroke engines.

Use the Online Calculator to Never Get It Wrong

Preparing the mix correctly is essential for the longevity and performance of a two-stroke engine. If you don't want to do calculations by hand or consult tables every time, our free 2-stroke mix calculator lets you enter the petrol quantity and the desired ratio to instantly get the exact amount of oil to add. If you are also interested in working out the cost of fuel for your trips, you can take a look at our fuel-consumption calculator or our guide to calculating fuel consumption. To work out the cost of a car trip, we also have the trip-cost calculator. If you are environmentally conscious, find out your carbon footprint and how to reduce it. And for any calculation involving percentages or proportions, we have the right tools for you.