Two-Stroke Premix Calculator
Oil for any fuel quantity at any ratio, in ounces and millilitres — plus a check on what you actually poured.
Check what you actually poured
Every common ratio
| Ratio | Per US gallon | Per litre | Where it belongs |
|---|---|---|---|
| 16:1 | 8.00 oz | 62.5 ml | Vintage and some older air-cooled engines. Very heavy on oil by modern standards, and it will foul plugs in an engine that was not designed for it. |
| 20:1 | 6.40 oz | 50.0 ml | Older two-strokes and some marine engines. Check the manual rather than assuming. |
| 24:1 | 5.33 oz | 41.7 ml | Some vintage road bikes and older karts. |
| 32:1 | 4.00 oz | 31.3 ml | The common figure for modern motocross and enduro engines being run hard. |
| 40:1 | 3.20 oz | 25.0 ml | A frequent compromise on trail bikes and small engines. |
| 50:1 | 2.56 oz | 20.0 ml | What most modern two-strokes specify, and what most pre-mixed fuel is sold at. |
| 100:1 | 1.28 oz | 10.0 ml | Rare, and almost always an oil-injection system rather than premix. Do not use it as a premix figure unless the manual says so. |
The number reads backwards
The ratio is fuel to oil. 50:1 means fifty parts fuel to one part oil, so a bigger first number means less oil. Going from 50:1 to 32:1 is adding oil, not removing it, which is the opposite of how the numbers look at a glance — and getting it backwards in either direction is expensive.
The arithmetic is just 128 divided by the ratio, because there are 128 fluid ounces in a US gallon. So 32:1 is exactly 4 ounces a gallon, 50:1 is 2.56, and 16:1 is a full 8.
More oil is a leaner mixture
This catches people out. Oil displaces gasoline in the same volume, so a can mixed at 32:1 contains less actual fuel per gallon than one mixed at 50:1. The engine sees a leaner air-to-fuel ratio, not a richer one — which is why changing premix ratio can call for a jetting change alongside it, and why "I added more oil to be safe" is not always the safe direction it sounds like.
Things worth knowing
- More oil means a LEANER fuel mixture, not a richer one. Oil displaces gasoline in the same volume, so going from 50:1 to 32:1 leaves less fuel per unit of air — which is why a ratio change sometimes needs a jetting change with it.
- The ratio is fuel to oil. 50:1 is fifty parts fuel to one part oil, so a bigger first number means LESS oil — the opposite of how it reads at a glance.
- Use the ratio in the manual. Guessing rich fouls plugs and cokes the exhaust; guessing lean seizes the engine, and only one of those is recoverable at the side of a trail.
- Mix in a can, never in the tank. Oil is denser than gasoline and will sit at the bottom of a tank quite happily, so the first part of the ride runs on almost pure oil and the last part on almost none.
- Pre-mixed fuel goes off. Ethanol-blended pump fuel can separate within weeks, and separated fuel takes the oil down with the water — mix what you will use, and write the ratio and the date on the can.
How to use
- Pick the ratio your manual specifies and enter the fuel quantity in gallons or litres.
- Read the oil to add, in fluid ounces and millilitres.
- If you have already poured, use the check section to see what ratio you really got.
Frequently asked questions
How much oil per gallon at 50:1?
2.56 fluid ounces, or 20 millilitres per litre. The arithmetic is simply 128 divided by the ratio, because there are 128 fluid ounces in a US gallon.
How much oil at 32:1?
Exactly 4 fluid ounces per US gallon, or 31.25 millilitres per litre. The round number is a coincidence of 128 dividing neatly by 32, and it is why 32:1 is easy to mix by eye.
Does a bigger ratio number mean more oil?
No, less. The ratio is fuel to oil, so 50:1 is fifty parts fuel to one part oil. Going from 50:1 to 32:1 is adding oil, which is the opposite of how the numbers read at a glance.
Is more oil a richer mixture?
It is a leaner one, which catches people out. Oil displaces gasoline in the same volume, so a can mixed at 32:1 holds less actual fuel per gallon than one at 50:1 — the engine sees a leaner air-to-fuel ratio, and a ratio change can call for a jetting change with it.
Can I mix it in the tank?
You should not. Oil is denser than gasoline and will happily sit at the bottom of a tank, so the first part of the ride runs on nearly pure oil and the last part on nearly none. Mix in a can, oil first, fuel on top.
How long does premixed fuel last?
Not long. Ethanol-blended pump fuel can separate within weeks, and separated fuel takes the oil down with the water. Mix what you will actually use and write the ratio and date on the can.
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