Why 2-stroke engines need oil in the fuel
Small 2-stroke engines in chainsaws, string trimmers, leaf blowers, dirt bikes and many outboard motors have no oil sump. The fuel passes through the crankcase on its way to the cylinder, and the oil mixed into it lubricates the crankshaft, bearings and piston. That is why the oil must be premixed with the gasoline in the right proportion.
The calculation
A ratio of 50:1 means 50 parts gasoline to 1 part oil:
Oil = gasoline ÷ ratio
In US units, 1 gallon is 128 fluid ounces, so at 50:1 you need 128 ÷ 50 = 2.56 fl oz per gallon.
Worked example
Mixing 1 gallon of gasoline at 50:1:
| Result | Value |
|---|---|
| Oil to add | 2.56 fl oz |
| In milliliters | 76 mL |
| Oil share | 2.0% |
| Total mix | 1.02 gal |
Quick reference
| Ratio | Per 1 gallon | Per 2.5 gallons |
|---|---|---|
| 100:1 | 1.28 fl oz | 3.2 fl oz |
| 50:1 | 2.56 fl oz | 6.4 fl oz |
| 40:1 | 3.2 fl oz | 8.0 fl oz |
| 32:1 | 4.0 fl oz | 10.0 fl oz |
| 25:1 | 5.12 fl oz | 12.8 fl oz |
Which ratio to use
Always check the engine’s manual or the label on the machine. As a rule:
- 50:1 is standard for most modern handheld equipment from major brands, with a good synthetic or semi-synthetic oil (JASO FD or ISO-L-EGD rated).
- 40:1 or 32:1 is common for older equipment and some dirt bikes.
- 100:1 appears only on specific oils and engines designed for it.
- Outboard motors with oil injection need no premix at all; others use TC-W3 oil, often at 50:1.
Using the right oil matters as much as the ratio. Do not use car engine oil, which is designed to stay in a sump, not to burn cleanly.
Mixing tips
- Pour the oil into an approved fuel can first, then add about half the gasoline.
- Close the can and shake to mix, then add the rest of the gasoline and shake again.
- Label the can with the ratio and the date.
- Use fresh gasoline with the octane the manual recommends. Ethanol blends above 10% (E15, E85) can damage small engines.
Mixed fuel degrades within about 30 days as gasoline oxidizes and ethanol absorbs moisture. A fuel stabilizer extends that, and canned premixed fuel with no ethanol keeps for years.