Two-stroke fuel mix calculator

Mixing two-stroke outboard fuel

A mix ratio is parts of petrol to one part of oil. At 50:1 — the most common ratio for modern pre-mix outboards — every 50 litres of petrol takes 1 litre of oil, so 20 litres takes 400 ml. At 40:1 the same 20 litres takes 500 ml, and at 100:1 only 200 ml.

Use the ratio in your manual

Always follow the ratio the manufacturer specifies for your motor, and use oil certified NMMA TC-W3 for water-cooled two-strokes — car or garden two-stroke oil is not the same thing. Many engines specify a richer break-in ratio for the first tanks. Running too little oil causes seizure; running far too much fouls plugs and smokes heavily.

Mixing method

Pour a little petrol into a clean, approved fuel can, add all the oil, then add the rest of the petrol and rock the can to combine — this avoids oil sitting on the bottom of the tank. Mix only what you will use in about 30 days; petrol with ethanol absorbs water and goes stale, and stale mix is behind a large share of the outboard problems we see.

Four-stroke outboards must never have oil added to the fuel. If you are moving from a pre-mix two-stroke to a modern four-stroke, fuel goes in neat and engine oil goes in the sump.