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Two-Stroke Fuel Mix Calculator

Get the exact oil for your two-stroke mix

How it is calculated

A ratio of 40:1 means 40 parts of petrol to 1 part of oil. The tool treats the fuel as litres and returns the oil in millilitres: oil_ml = fuel × 1000 / ratio. It then converts to US fluid ounces with oil_oz = oil_ml / 29.5735. Enter only the first number of the ratio (40 for 40:1). Both outputs have two decimals. Fuel and ratio must be above zero, otherwise nothing is shown. The tool does not know which ratio your engine needs, so take it from the manual or the oil label, and measure petrol in litres: US gallons must be converted first.

Worked example

You are filling a 5-litre can for a trimmer that needs 40:1. Oil: 5 × 1000 / 40 = 125.00 ml. In fluid ounces: 125 / 29.5735 = 4.23 oz. Pour the oil into the can first, add about half the petrol, shake, then top up with the rest of the 5 litres.

Try this example →

Common mistakes

  • Entering the fuel in US gallons or imperial gallons. The field is litres, so 1 gallon typed as 1 gives a quarter of the oil needed.
  • Typing the oil share (2.5 for 2.5%) instead of the ratio number (40). 40:1 is about 2.5% oil, but the field wants 40.
  • Confusing 40:1 with 1:40, or mixing up which side is oil. The first number is petrol parts, the oil is always the small one.
  • Reading the millilitre result as ounces, or the reverse. The two lines are the same amount in different units, not two additions.
  • Using the ratio from an old engine or a different brand of oil. Check what the engine maker and the oil label say.

About this calculator

Two-stroke engines have no separate oil sump, so oil must be pre-mixed into the petrol at a specific ratio — this calculator computes oil needed = fuel volume ÷ ratio (a 50:1 ratio on 5 liters of petrol needs 100 ml of oil), scaling to whatever ratio the engine manufacturer specifies.

Owners of chainsaws, string trimmers, outboard motors, and mopeds use this before every fill-up, since running a two-stroke engine on straight petrol destroys it within minutes from lack of lubrication, while too much oil fouls spark plugs and leaves excess carbon deposits — getting the ratio exactly right matters more than with any four-stroke engine, where oil and fuel are always kept separate.

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