A scored piston is expensive evidence that fuel handling matters. A wrong oil ratio can reduce lubrication, but it is only one possible cause of a damaged two-stroke engine. This calculator removes the measuring error from one part of the job: adding the correct volume of oil to a known amount of petrol.
Enter the amount of petrol before oil is added, choose the ratio printed in your machine’s manual, and the tool gives you the oil volume in metric and US units. Do not guess a richer or leaner oil mix as a workaround for an engine problem.
Quick answer
50:1 needs 20 ml of two-stroke oil per litre of petrol, or about 2.6 US fl oz per US gallon. 40:1 needs 25 ml per litre, or 3.2 US fl oz per US gallon.
| Petrol amount | 50:1 oil | 40:1 oil | 32:1 oil | 25:1 oil |
|---|---|---|---|---|
| 1 litre | 20 ml | 25 ml | 31 ml | 40 ml |
| 2 litres | 40 ml | 50 ml | 63 ml | 80 ml |
| 5 litres | 100 ml | 125 ml | 156 ml | 200 ml |
| 10 litres | 200 ml | 250 ml | 313 ml | 400 ml |
| 1 US gallon | 2.6 fl oz | 3.2 fl oz | 4.0 fl oz | 5.1 fl oz |
Rounded for practical measuring. For small batches, use a marked mixing bottle or measuring syringe rather than estimating from a bottle cap.
Fuel-mixing limits
This calculator does not diagnose a poor-running engine. If a saw, trimmer, blower or mower has overheated, lost power, rattled, or seized, check the fuel quality, cooling system, air filter, carburetion and service condition rather than changing the oil ratio by instinct. See our chainsaw maintenance guide for the wider inspection routine.
Getting the fuel-to-oil ratio mathematically correct is only the baseline requirement for engine longevity. Modern petrol frequently contains ethanol blends (such as E10), which are hygroscopic and absorb moisture from the atmosphere. When left sitting, this causes phase separation, the water and ethanol mixture drops out of suspension. If the engine ingests this layer, it runs without lubrication, leading to catastrophic failure regardless of your mixture accuracy.
The physical volume of oil is also irrelevant if the chemical quality is substandard. Mixing uncertified oil at a perfect 50:1 ratio will still result in heavy carbon deposits that score pistons and stick rings. It is imperative to use two-stroke oils that meet modern certifications, such as JASO FD or ISO-L-EGD, which guarantee the high detergency required by air-cooled equipment.
Finally, the operational lifespan of mixed two-stroke fuel is severely limited. Once petrol and oil are combined, the volatile compounds in the fuel evaporate, altering combustion characteristics. Any fuel mixture older than 30 days should be considered stale and unfit for use in high-tolerance machinery. Stale mix must be properly disposed of, rather than forced through a tool to save a few cents.
