This gas oil mix ratio calculator converts a fuel volume and a mix ratio into the exact amount of two-stroke oil to add, in millilitres or fluid ounces, and can also run backwards to tell you what ratio a mixture you have already poured actually produces. It takes the ratio as an input for a deliberate reason, explained below.
Arb Digital publishes it inside a large free tools library. Before anything else, the important part: the correct ratio for any two-stroke engine comes from that engine manufacturer's own manual, and nowhere else. Running a mixture that is too lean on oil starves the engine of lubrication and can destroy it; running it far too rich fouls plugs and clogs exhaust ports. This page will do the arithmetic perfectly for whatever ratio you give it, and it deliberately does not publish a table of recommended ratios by brand or machine type, because the only correct source is the documentation that came with your equipment.
What This Gas Oil Mix Ratio Calculator Does
In its default mode you enter a fuel volume in litres, US gallons or imperial gallons, plus the ratio your manual specifies, and the headline figure is the oil quantity to add in whichever unit you have selected. The supporting grid gives oil per litre of fuel, the oil percentage, the total volume of the finished mixture and the resulting ratio as a check.
Switching to reverse mode changes the question. Enter the fuel volume and the oil you have already put in, and the tool reports the ratio that mixture actually is. This is the mode to use when a can has been made up from a partly used bottle, or when you are trying to work out whether topping up a half-full can with plain fuel has pushed the mixture out of specification.
The bar display shows fuel, oil and total mix on the same scale, which is a useful visual reminder of how small the oil quantity is. At 50:1 the oil is under two per cent of the fuel volume, and an error of a few millilitres in a small can is proportionally large.
How to Use It
- Find the ratio in your manual. It is normally printed on the fuel cap, in the specification table, or both. If the machine and the manual disagree, contact the manufacturer rather than guessing.
- Enter the fuel volume you are actually mixing. Mix what you will use within a few weeks rather than filling a large can you will still be working through months later.
- Choose the output unit that matches the measuring jug or the graduations on the oil bottle you are pouring from.
- Mix in a clean container. Pour part of the fuel, add the measured oil, then the rest of the fuel, and agitate before filling the machine.
- Label the can. Write the ratio and the date on it. An unlabelled can of pre-mix is the most common cause of the wrong fuel going into the wrong machine.
The Formula / How It's Calculated
A mix ratio written as 50:1 means fifty parts fuel to one part oil by volume, so the oil quantity is simply fuel volume ÷ ratio. It is not fifty-one parts total, and the oil is not added as a percentage of the finished mixture — a distinction that matters at rich ratios such as 16:1 where the difference is noticeable.
Worked example. One US gallon of fuel at 50:1. A US gallon is 3,785.41 millilitres, so the oil quantity is 3,785.41 ÷ 50 = 75.71 millilitres. Converting to US fluid ounces at 29.5735 ml each gives 2.56 fl oz, which is the familiar figure printed on most oil bottles for a one-gallon mix.
In metric the arithmetic is easier to do in your head: 5 litres at 50:1 is 5,000 ÷ 50 = 100 ml of oil, and the oil is 2 per cent of the fuel volume. At 40:1 the same 5 litres needs 125 ml, and at 25:1 it needs 200 ml.
Reverse mode divides the other way: ratio = fuel volume ÷ oil volume, with both in the same units. Five litres of fuel with 125 ml of oil in it is 5,000 ÷ 125 = 40, so that can is 40:1 — which is a problem if the machine it is destined for calls for 50:1, and a much bigger problem the other way round.
Why This Page Does Not List Ratios by Brand
Charts of "recommended" ratios by manufacturer circulate widely and they are genuinely dangerous, for three reasons. Ratios differ between models from the same maker; they differ between engine generations, so a chart that was right for a machine built in one decade is wrong for its replacement; and they depend on the oil specification being used, because an oil formulated to one performance standard is not interchangeable with one formulated to another at the same ratio.
The consequence of getting it wrong is asymmetric. Too little oil removes lubrication from the bearings and cylinder wall, and a two-stroke seizure is usually terminal rather than repairable. Too much oil is far less destructive but still causes plug fouling, carbon build-up on the piston crown and exhaust port, smoke and lost power.
So the ratio is an input on this page, not an output, and that is deliberate. Read the number off your machine or its manual, type it in, and let the calculator do the part it can do reliably.
Fuel Choice, Ethanol and Storage Life
Mixed two-stroke fuel does not keep. The petrol component oxidises and the light fractions evaporate, and a can mixed months ago can be noticeably harder to start on and worse to run on than fresh fuel. Mixing small quantities frequently is better practice than mixing a large batch once.
Ethanol content makes this worse. Ethanol-blended petrol attracts water, and in a partly full can that moisture can separate out over time, taking dissolved ethanol with it and leaving a layer at the bottom of the container. The US Department of Energy's Alternative Fuels Data Center describes the common ethanol blends available at the pump and which equipment each is approved for — a point worth checking, because approval for a blend in a car says nothing about approval in a chainsaw or an outboard.
Small non-road engines are also a regulated category in their own right, and the US Environmental Protection Agency maintains emission standards for small equipment and tools. Those rules are one reason manufacturers are specific about fuel and oil requirements: an engine certified on a particular fuelling arrangement is expected to be run on it.
Measuring Small Volumes Accurately
The oil quantities involved are small enough that casual measurement introduces real error. At 50:1, a one-litre can needs 20 millilitres of oil. Being 5 millilitres out — roughly a teaspoon — makes it a 40:1 mix instead, a 25 per cent error in oil content from a quantity most people would pour by eye without thinking.
Use a graduated measuring container rather than the cap of the oil bottle, and read the graduation at eye level with the container on a flat surface. Two-stroke oil is viscous, so allow it to drain properly and rinse the measure with a little of the fuel to recover what clings to the sides. Pre-measured single-dose bottles sized for a specific fuel quantity remove the measuring step entirely, and are worth the premium for anyone who mixes infrequently.
If you are working between imperial and metric measures — a common situation with an American machine and a European oil bottle — convert once and write the result down rather than converting repeatedly in your head. The metric to SAE converter handles the unit work, and the ratio calculator and dilution ratio calculator cover proportional mixing in general.
Where a Ratio Mistake Actually Shows Up
The symptoms of a mixture problem are not always obvious, and they are frequently misread as ignition or carburettor faults. A mixture too rich in oil tends to produce heavy blue smoke, a plug that fouls repeatedly, sluggish response at high revs and gradual carbon accumulation. Those are inconvenient rather than catastrophic and are usually reversible with a clean plug and fresh correct fuel.
A mixture too lean in oil gives far fewer warnings. It may run cleanly and even feel strong right up to the point of failure, which typically appears as a seizure under load. This asymmetry is why, when there is genuine doubt about which of two ratios applies, the safe direction is not "somewhere in the middle" — it is to stop and confirm the correct figure from the manufacturer.
One more scenario worth naming: mixed fuel accidentally used in a four-stroke engine, or straight petrol accidentally used in a two-stroke. The first fouls plugs and smokes; the second removes lubrication entirely and is the classic way a strimmer or chainsaw is destroyed in a single afternoon. Labelled cans prevent both. For related equipment maths, the generator size calculator and the fuel cost calculator are on the site.
Arb Digital builds local search visibility for equipment dealers and trade services, and the free tools library is open to everyone.
SEO Services Talk to Arb DigitalCommon Mistakes to Avoid
- Taking the ratio from a chart instead of the manual — ratios vary by model, by engine generation and by oil specification, and only your manufacturer's documentation is authoritative.
- Treating the ratio as parts of finished mixture — 50:1 means fifty parts fuel to one part oil, not one part oil in fifty-one of mix.
- Mixing in the machine's tank — the oil and fuel do not blend reliably, and the worst-mixed fuel reaches the engine first.
- Topping a part-used can with plain fuel — it dilutes the mixture to an unknown ratio, which is exactly what the reverse mode on this page is for.
- Keeping mixed fuel for months — mixed petrol degrades, and ethanol blends can separate in a part-full container.
Related Free Tools From Arb Digital
Handle proportional mixing generally with the ratio calculator and the dilution ratio calculator, convert units with the metric to SAE converter, and work out running costs with the fuel cost calculator. On the engine side, the air fuel ratio calculator covers combustion mixture and the engine compression ratio calculator covers geometry. Everything else is in the free online tools hub.
Frequently Asked Questions
A US gallon is 3,785.41 millilitres, so at 50:1 the oil quantity is 75.71 millilitres, or 2.56 US fluid ounces. An imperial gallon is larger at 4,546.09 millilitres and needs 90.92 millilitres.
Only the manufacturer's manual for your specific model can answer that. Ratios vary between models, between engine generations and with the oil specification used, so this page takes the ratio as an input rather than recommending one.
No. It means fifty parts of fuel to one part of oil by volume, so the oil quantity is the fuel volume divided by fifty. The distinction becomes significant at rich ratios such as 16:1.
The engine loses lubrication at the bearings and cylinder wall, and the usual outcome is a seizure under load that is rarely economically repairable. Too little oil gives far less warning than too much, which is why the correct figure matters.
Typically heavy smoke, repeated spark plug fouling, carbon build-up on the piston and exhaust port and reduced power. It is unpleasant rather than destructive and usually clears with a new plug and correct fresh fuel.
Not long. Petrol oxidises and its lighter fractions evaporate, and ethanol blends can attract water and separate in a part-full container. Mixing small quantities regularly is better practice than storing a large batch.
Yes. Switch the calculator to reverse mode, enter the fuel volume and the oil already added, and it divides one by the other to report the ratio that mixture actually is.
The arithmetic is the same, but the correct ratio can differ with oil specification, so use the ratio your equipment manual states for the type of oil you are using rather than assuming one figure covers everything.
This tool performs a volume calculation only. The correct fuel and oil specification for any engine comes from that manufacturer's own documentation, and using an incorrect mixture can damage or destroy a two-stroke engine.
