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2-Stroke Fuel Mix Calculator

Find the exact oil amount for your 2-stroke petrol mix ratio.

How it is calculated

The ratio is read as parts of petrol to one part of oil, so a 50 means 50:1. The oil needed is oil (L) = petrol (L) ÷ ratio, shown in litres and in millilitres (× 1,000). The finished mixture is total = petrol + oil. Petrol must be in litres, and both the ratio and the amount must be greater than zero. The oil is added on top of the petrol you enter, not taken out of it. A larger ratio number means a leaner mix with less oil; always follow the ratio and oil type in the engine's manual.

Worked example

An engine calls for 50:1 and you want to mix 5 litres of petrol. Oil = 5 ÷ 50 = 0.10 L, which is 0.10 × 1,000 = 100 mL. The total mixture is 5 + 0.10 = 5.10 L. So add 100 mL of oil to 5 L of petrol to get 5.10 L of ready fuel.

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Common mistakes

  • Entering the amount in millilitres or US gallons. The tool assumes litres, so a gallon figure gives the wrong oil volume.
  • Using the percentage in place of the ratio. 2% oil is a 50:1 mix, so type 50, not 2.
  • Believing a smaller ratio number means less oil. 25:1 contains twice as much oil as 50:1, which can foul spark plugs and cause smoke.
  • Expecting the total to equal the petrol you entered. The oil is extra, so the finished mix is slightly larger (5.10 L here), and a tank sized for 5 L may overflow.

About this calculator

This calculator works out how much 2-stroke oil to mix into a given volume of petrol for a chosen ratio, such as 50:1, 40:1, or 32:1 — common ratios specified by chainsaw, trimmer, and outboard motor manufacturers. Two-stroke engines burn oil together with fuel to lubricate the piston and crankshaft, so getting the ratio wrong can cause excess smoke and fouled plugs (too much oil) or engine seizure from inadequate lubrication (too little).

Chainsaw and trimmer owners, boaters with 2-stroke outboards, and small-engine mechanics use it whenever they mix a new batch of fuel, since manufacturers specify different ratios for different equipment. If needed, the oil quantity can be shown in milliliters or ounces.

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Comments (1)

  • Editorial team

    Treat the ratio field as litres of petrol per one part of oil, so 50 means 50:1. Check your engine's manual first, as the ratio depends on the oil type and the engine, and running leaner than the manual says can damage it. Measure the oil in a graduated cup rather than by eye, because a few millilitres matter in small 1-litre batches. Mix in the right order and shake well. Which engine are you fuelling? Tell us the ratio you use.

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