Two-Stroke Oil Ratios Explained: 50:1, 40:1, 32:1 and 25:1
Chainsaws, strimmers, leaf blowers, outboard motors, dirt bikes and classic scooters often run on two-stroke engines that need oil mixed into the fuel. Getting the mix right is simple once you understand the ratio, and getting it wrong is one of the most common reasons these engines smoke, foul plugs or seize. This guide explains the numbers and the practical side of mixing.
Why two-strokes need oil in the fuel
A four-stroke engine has an oil sump and a pump. Most small two-stroke engines do not: the fuel-air mixture passes through the crankcase on its way to the cylinder, and the oil carried in the fuel lubricates the crankshaft bearings, connecting rod and piston. The oil then burns with the fuel. Some larger two-strokes have a separate oil tank and pump (oil injection), in which case you do not premix unless the manual says so.
What the ratio means
A ratio of 50:1 means 50 parts of petrol to 1 part of oil. Divide the amount of fuel by the ratio to get the oil:
Oil = fuel ÷ ratio
5,000 ml ÷ 50 = 100 ml
| Ratio | Oil % | Per litre | Per US gallon |
|---|---|---|---|
| 50:1 | 2% | 20 ml | 2.56 fl oz (76 ml) |
| 40:1 | 2.5% | 25 ml | 3.2 fl oz (95 ml) |
| 32:1 | 3.1% | 31 ml | 4 fl oz (118 ml) |
| 25:1 | 4% | 40 ml | 5.12 fl oz (151 ml) |
For any other amount, use the two-stroke oil mix calculator, which also handles UK gallons (about 20% larger than US gallons).
Which ratio is right?
Always use the ratio in your owner's manual, or printed on the fuel cap or tank. Modern garden tools and many scooters run at 50:1 with good synthetic or semi-synthetic oil. Some brands specify 40:1. Older manuals from the days of mineral oils often say 32:1 or 25:1, and some engine builders specify a richer mix for running in a rebuilt engine. If your manual is lost, the manufacturer's website or dealer can tell you.
More oil does not mean more protection. An over-rich mix leaves carbon on the piston, plug and exhaust, causes smoke and can make the engine run slightly lean because oil replaces some fuel. Too little oil risks overheating and a seized piston. The design ratio is the safe one.
Choosing the oil
- Air-cooled engines (chainsaws, scooters, most garden tools): use a two-stroke oil rated JASO FB, FC or FD, or ISO-L-EGC/EGD. API TC is an older but still common rating.
- Water-cooled outboard motors: use oil rated NMMA TC-W3. It is formulated for cooler-running marine engines and is generally not recommended for air-cooled engines.
- Never use four-stroke car or motorcycle oil in a premix.
- Some oils are labelled "pre-mix" with a measuring chamber in the bottle; still check that the amount matches your ratio.
How to mix
- Use a clean, approved fuel can, and fresh petrol of the octane your manual recommends.
- Pour in about half the petrol, then all of the measured oil.
- Add the rest of the petrol, close the can and shake well.
- Label the can with the ratio and date. Shake briefly before each refill, as oil can settle.
Measure the oil with a graduated bottle or syringe rather than a bottle cap. Many manufacturers recommend using mixed fuel within about a month, or adding a stabiliser for longer storage. Old mix can cause hard starting and gummed carburettors.
Worked examples
- A 1-litre chainsaw tank at 50:1: 20 ml of oil.
- A 5-litre can at 40:1: 125 ml.
- One US gallon at 50:1: 2.56 fl oz, about 76 ml.
- A classic scooter taking 8 litres at 2%: 160 ml.
Common mistakes
- Confusing 1:50 with 50:1: they mean the same here, one part oil to fifty parts fuel.
- Using US-gallon charts with UK gallons (or the reverse).
- Putting premix in an oil-injected engine without removing or disabling the pump as the manual describes.
- Mixing in the tank by pouring oil in first and fuel on top without shaking; the oil may not disperse evenly.