How the ratio works
Two-stroke engines have no separate oil sump. The oil that lubricates the crankshaft, connecting rod and piston travels with the fuel, so it has to be mixed into the petrol (premix) or pumped in by an oil-injection system. The mix ratio tells you how many parts of petrol go with one part of oil: 50:1 means 50 parts petrol to 1 part oil.
Oil = petrol volume ÷ ratio
Oil percentage = 100 ÷ ratio
So 50:1 is the same as 2% oil, 40:1 is 2.5%, 32:1 is about 3.1% and 25:1 is 4%. A smaller ratio number means more oil.
Quick reference table
| Petrol | 100:1 | 50:1 | 40:1 | 32:1 | 25:1 |
|---|---|---|---|---|---|
| 1 litre | 10 ml | 20 ml | 25 ml | 31 ml | 40 ml |
| 5 litres | 50 ml | 100 ml | 125 ml | 156 ml | 200 ml |
| 10 litres | 100 ml | 200 ml | 250 ml | 313 ml | 400 ml |
| 1 US gallon | 1.28 fl oz | 2.56 fl oz | 3.2 fl oz | 4 fl oz | 5.12 fl oz |
| 1 UK gallon | 45 ml | 91 ml | 114 ml | 142 ml | 182 ml |
Worked examples
A 5-litre can at 50:1
5 litres is 5,000 ml. 5,000 ÷ 50 = 100 ml of oil. This is the most common mix for modern chainsaws, strimmers and many classic scooters run on today's oils.
One US gallon at 40:1
A US gallon is 3,785 ml. 3,785 ÷ 40 = 94.6 ml, which is 3.2 US fluid ounces (one US fl oz is 29.57 ml). At 50:1 the same gallon needs 2.56 fl oz, and at 32:1 exactly 4 fl oz.
A 2-litre break-in mix at 25:1
Some rebuilt engines are run in on a richer mix for the first tank or two. 2,000 ml ÷ 25 = 80 ml. Only do this if your engine builder or manual recommends it.
Which ratio should you use?
Always follow the manufacturer's recommendation, which you will find in the owner's manual or on the fuel cap or tank. As a rough guide:
- 50:1 is standard for most modern garden and forestry tools and many motorcycles and scooters when used with a good synthetic or semi-synthetic oil.
- 40:1 is common for some brands of power tools and outboard motors.
- 32:1 and 25:1 appear in older manuals written for oils of the 1970s and 1980s, and in some racing or break-in situations.
- 100:1 is only for engines and oils specifically rated for it.
More oil is not automatically safer. Too much oil can foul the spark plug, leave carbon on the piston and exhaust, and make the mixture slightly leaner, because oil takes the place of some petrol. Too little oil risks a seized piston. The right ratio is the one the engine was designed and tested with.
Choosing the oil
- Use an oil made for air-cooled two-stroke engines if your engine is air-cooled. Look for JASO FB, FC or FD, ISO-L-EGC/EGD, or API TC on the bottle.
- Outboard (TC-W3) oil is designed for water-cooled marine engines that run cooler. Do not use it in air-cooled engines unless the manufacturer allows it.
- Never use four-stroke engine oil in a premix. It is not designed to burn cleanly with the fuel.
How to mix correctly
- Use a clean, approved fuel can and fresh petrol.
- Pour in about half of the petrol.
- Add the full measured amount of oil. A small measuring bottle or syringe makes this accurate.
- Add the rest of the petrol, close the can and shake it well.
- Label the can with the ratio and the date. Shake again briefly before each refill.
Mixed fuel ages faster than plain petrol. Many manufacturers recommend using it within about a month, or adding a fuel stabiliser if it will be stored longer.
Common mistakes
- Mixing up 1:50 and 50:1. Both mean the same thing here: one part oil to fifty parts petrol.
- Using a US-gallon chart for UK gallons. A UK gallon is about 20% bigger, so it needs about 20% more oil.
- Guessing with the bottle cap. Caps vary a lot in size. Measure.
- Premixing into an oil-injected engine. If your machine has an oil tank and pump, it mixes the oil itself. Only premix if the manual says so, for example when the pump has been removed.
Frequently asked questions
How much oil do I add to 1 litre of petrol at 50:1?
20 ml. Divide 1,000 ml by 50.
How many ounces of 2-stroke oil per gallon at 50:1?
2.56 US fluid ounces per US gallon. At 40:1 it is 3.2 fl oz, and at 32:1 it is 4 fl oz.
Is 50:1 the same as 2%?
Yes. 100 ÷ 50 = 2, so the oil makes up about 2% of the petrol volume.
What happens if I put too much oil in the mix?
The engine may smoke more, foul spark plugs and build carbon faster, and it can run slightly lean. A one-off small error is rarely harmful, but stick to the recommended ratio.
Can I use car engine oil in a two-stroke?
No. Car oil is made for four-stroke engines with a sump. It does not burn cleanly in a premix and can damage a two-stroke engine.
Sources
- JASO M345 two-stroke engine oil standard (FB, FC, FD performance classes), Japanese Automotive Standards Organization.
- ISO 13738 specification for two-stroke gasoline engine oils (EGB, EGC, EGD).
- Typical manufacturer owner's manuals for chainsaws, outboard motors and two-stroke scooters (recommended premix ratios).
Last reviewed 5 October 2026. Results are estimates; see our disclaimer.