How this calculator works
A two-stroke engine mixes oil into the fuel because it has no separate sump for the crankshaft and cylinder lubrication. A 50:1 ratio means 50 parts of gasoline to one part of oil by volume. Dividing the gasoline volume by the ratio gives the oil volume, and multiplying the oil volume by the ratio gives the gasoline volume. The oil percentage is one part divided by the total parts, so 50:1 is 1/51, or about 1.96 percent of the finished mixture. The ratio is based on volume, not mass, and it assumes the oil and gasoline are measured at ordinary outdoor temperatures.
Formula
ratio = gas volume / oil volume. Oil volume = gas volume / ratio. Gas volume = oil volume x ratio. Oil percentage = 1 / (ratio + 1).
Worked example
For 5 litres of gasoline at 50:1, add 5 / 50 = 0.1 litres of oil, which is 100 mL. The oil is 1 / 51 = 1.96 percent of the mixture. At 40:1 the same 5 litres needs 0.125 litres of oil, or 125 mL.
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Frequently asked questions
What ratio should I use?
Follow the engine maker. Common values are 50:1 for many modern two-stroke engines, 40:1 for older equipment and 32:1 for some high-performance engines. Using too little oil risks seizure, while too much oil fouls plugs and makes smoke.
Can I use any two-stroke oil?
Use oil rated for air-cooled or water-cooled two-stroke engines as the maker specifies. Outboard, chainsaw and motorcycle oils are not always interchangeable, and four-stroke oil should not be used as a substitute.
Is the ratio by volume or weight?
By volume, as written on fuel cans and oil bottles. Measure the fuel and the oil with a clean container rather than estimating from the size of the tank.
Does ethanol fuel change the ratio?
The ratio stays the same, but ethanol attracts water and can separate during storage. Mix only what you will use in a few weeks and follow the equipment maker's guidance on ethanol content.