Gas Oil Ratio Calculator (2-stroke mix 50:1, 40:1, 32:1)

Understanding the two-stroke fuel mix calculator — concept and principle

This calculator tells you how much two-stroke oil to add to petrol (gasoline) for a given mix ratio, such as 50:1 or 40:1, in millilitres and fluid ounces. Owners of chainsaws, leaf blowers, strimmers, outboard motors and older mopeds use it every time they mix fuel.

Petrol divided by the ratio: with the default inputs the result is 100 mL oil (≈ 3.4 US fl oz · 5.1 L mix).

How it is calculated

Oil = petrol ÷ ratio

Petrol is first converted to litres (US gallon = 3.785411784 L, UK gallon = 4.54609 L).
Oil in ml = litres × 1,000 ÷ ratio; US fl oz = ml ÷ 29.5735; UK fl oz = ml ÷ 28.4131.
Total mix = petrol + oil; oil share = oil ÷ total.
Oil per litre of petrol = 1,000 ÷ ratio ml.

The principle

A two-stroke engine has no separate oil sump; the oil travels with the fuel and lubricates the crankshaft, bearings and piston as the mixture passes through the crankcase. The ratio means parts of petrol to one part of oil by volume, so 50:1 is 50 litres of petrol to 1 litre of oil. Because the oil is added on top, the finished mix is slightly more than the petrol you started with, and the oil share is 1 ÷ (ratio + 1), a little less than 1 ÷ ratio.

Worked example

Default inputs: 5 litres of petrol, ratio 50:1.

Oil = 5 ÷ 50 = 0.1 L = 100 ml.
In fluid ounces = 100 ÷ 29.57 = 3.4 US fl oz, or 100 ÷ 28.41 = 3.5 UK fl oz.
Total mix = 5 + 0.1 = 5.1 litres.
Oil share = 0.1 ÷ 5.1 = 1.96%; per litre of petrol = 1,000 ÷ 50 = 20 ml.

Each point is the calculator re-run with only “Mix ratio — petrol parts to 1 part oil” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Mix ratio — petrol parts to 1 part oil	Two-stroke oil to add (ml)
12.5	400
25	200
37.5	133
50	100
62.5	80
75	67
100	50

How to read the result

Measure the oil with a marked bottle or syringe, add it to fresh petrol in a clean, approved can, and shake well before each use. Mix only as much as you will use within a few weeks, because petrol ages. The table on the page helps with other common ratios such as 25:1, 32:1 and 40:1.

Common mistakes

Using a ratio from memory instead of the engine manual.
Mixing US and UK gallons or fluid ounces.
Using four-stroke engine oil instead of two-stroke oil.
Putting mixed fuel into a four-stroke engine.

Keep in mind: Always use the ratio in your engine's manual. Too little oil can seize the engine; too much causes smoke and carbon deposits. Mix fuel outdoors, away from flames.

Frequently asked questions

How much oil for 5 litres at 50:1?

100 ml of two-stroke oil.

How much oil per gallon for 50:1?

About 2.6 US fl oz per US gallon.

What does 40:1 mean?

40 parts of petrol to 1 part of oil by volume, or 25 ml of oil per litre.

Sources: NIST Special Publication 811 (2008), Appendix B (1 US gallon = 3.785411784 L = 128 US fl oz; 1 UK gallon = 4.54609 L = 160 UK fl oz)

Checked 9 Nov 2026. Volume mixing ratio (parts of fuel per part of oil); exact units from NIST SP 811 (2008), Appendix B (1 US gallon = 3.785411784 L = 128 US fl oz; 1 UK gallon = 4.54609 L = 160 UK fl oz).

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