Two-Stroke Fuel Mix Ratio
The ratio below is the one printed on your machine or in its handbook. This page does not suggest one, does not know your engine and has no opinion about which oil you use — it measures out the volumes once you have supplied the ratio that engine was built for.
Petrol to oil, written as it appears: 50:1, 40:1, 25:1. A percentage such as 2% works too.
Oil to add
100 ml
Mixed at 50:1 by volume — 50 of petrol to 1 of oil
- Oil in millilitres100 ml
- Oil in US fluid ounces3.38 US fl oz
- Oil in imperial fluid ounces3.52 imp fl oz
- Petrol in litres5 L
- Finished mix5.1 L
How this was worked out
- What the ratio says50:1 means 50 parts petrol to 1 part oil, by volume
- The petrol you measured5 L = 5 L
- Petrol divided by the ratio5 ÷ 50 = 0.1 L = 100 ml of oil
- Petrol and oil together5 + 0.1 = 5.1 L in the can
- The same ratio, read backwards100 ml of oil × 50 = 5 L of petrol
Working back from a bottle
Two-stroke oil is sold in fixed sizes, so the question often runs this way round.
Petrol that bottle makes up
5 L
At the same ratio as above
- Petrol in US gallons1.32 US gal
- Petrol in imperial gallons1.1 imp gal
- Mix it makes5.1 L
Which number is which: 50:1 is fifty parts petrol to one part oil, so the oil is a fiftieth of the petrol and five litres takes 100 ml. A larger left-hand number therefore means less oil, not more — 25:1 is double the oil of 50:1 for the same fuel. Getting it the wrong way round is expensive in one direction and messy in the other: too little oil and the engine runs dry and picks up a seizure, too much and it smokes, fouls plugs and cokes the exhaust port. The proportion is a property of the engine, so the handbook or the decal on the machine decides it, not a website and not the last machine you owned.
A ratio and a percentage describe the same mix from opposite ends. 2% on an oil bottle means oil at 2% of the petrol, which is 50:1; measured against the finished mix instead it is 1.96%, because the oil is in there too. Both readings are shown above, and the gap between them is 2% at 50:1 and 4% at 25:1 — small, but not nothing, and worth knowing which one a bottle is quoting.
Fluid ounces are split the same way gallons are. A US fluid ounce is 29.57 ml and an imperial one is 28.41 ml, so a bottle marked in ounces is not necessarily marked in yours. Both are given above, and millilitres are given first because a measuring jug settles the argument.
Volumes only, mixed at the ratio you typed. Nothing here knows about oil grades, about fuel-stabiliser dose rates, about ethanol content or about how long a mixed can stays good — and nothing here is a substitute for what is printed on the machine.
The ratio belongs to the engine, not to this page
A two-stroke engine has no sump and no oil pump. Its bearings and cylinder wall are lubricated by oil carried in the fuel itself, which means the proportion of oil is a design parameter of that particular engine, chosen by whoever built it and printed on the machine or in its handbook. Chainsaws, strimmers, older outboards, dirt bikes and generators are specified all over the range, and one manufacturer will use different proportions for different models in the same catalogue.
So this tool asks for the ratio and does not offer one. Getting it wrong is expensive in both directions: too little oil and the piston runs dry, picks up on the bore and seizes, often within one tank; too much and the engine smokes, glazes the bore, fouls plugs and clogs the exhaust port with carbon.
What a ratio like 50:1 actually says
It is a volume ratio, petrol first: fifty measures of petrol to one measure of oil. The oil is therefore a fiftieth of the petrol, not a fiftieth of the finished mix, and the finished mix is 51 parts. That distinction is small — a shade under two per cent at 50:1, around four at 25:1 — but it is the difference between two answers and this page tells you which one it gave.
Bottles sometimes state a percentage rather than a ratio, and the two convert cleanly: 2% oil in the petrol is the same instruction as 50:1, 2.5% is 40:1, and 4% is 25:1. Ratios also appear written the other way round in casual use, and a reversed figure typed here is flagged rather than quietly corrected, because a page that silently reinterprets your input is a page you cannot check.
Measuring it out, in units that are not interchangeable
Five litres of petrol at 50:1 takes 100 ml of oil. One US gallon at the same ratio takes 75.7 ml; one imperial gallon takes 90.9 ml, because the imperial gallon is the larger container by about a fifth. Fluid ounces are split along the same fault line — a US fluid ounce is 29.57 ml and an imperial one is 28.41 ml — so a jug marked in ounces is not necessarily marked in your ounces.
That is why millilitres come first in the results above, with both fluid ounces beside them. A graduated jug or the measuring cap that comes with the oil is worth using; eyeballing a splash into a can is how a mix ends up 30% out. The second calculation runs the question backwards, which is how it usually arrives in practice: a 100 ml sachet at 50:1 makes up five litres of petrol, so that is how much to put in the can.
Mixing and keeping it
Mix in a clean, marked fuel can rather than in the machine’s tank, where nothing will combine properly. Put some of the petrol in first, add the measured oil, then the rest of the petrol, and rock the sealed can to fold the two together. Label the can with the ratio in large letters: an unlabelled can of premix looks exactly like an unlabelled can of straight petrol, and the two are not interchangeable in either direction.
Mixed fuel does not keep as well as neat petrol. Modern pump fuel containing ethanol can absorb water and separate over time, and stale premix is a common reason a machine will not start in spring. Mix what you will use, keep it sealed, and run the machine dry before a long layup rather than leaving old mix in a carburettor.
Lost handbooks, extra oil and oil injection
I have lost the handbook. What ratio should I use?
Look before you guess: the figure is often on a decal beside the fuel cap, moulded into the tank, or in the manual on the manufacturer’s support site, which usually keeps them for decades. A dealer can look up a serial number. Nothing here can tell you what your engine was built for.
Is extra oil a safety margin?
No. A rich mix does not lubricate better; it burns dirty, coats the plug and the exhaust port with carbon, and can lean the fuelling because oil displaces petrol in the charge. The specified proportion is the safety margin, and it was chosen with a particular oil specification in mind.
Can I use ordinary engine oil?
No. Two-stroke oil is formulated to burn with the fuel and leave little residue, and the maker names a specification alongside the ratio. Oil meant for a sump will coke up a two-stroke quickly.
My machine has oil injection. Do I still premix?
Only if the manual says so. Machines with an oil pump take straight petrol and meter their own oil, and adding premix on top over-oils them. Some manufacturers ask for premix during a running-in period, which is exactly the sort of detail the handbook carries.
Does the ratio change with the oil I buy?
It can. Some synthetic oils are approved for a leaner mix in specific engines, and that approval comes from the engine maker rather than from the bottle. Where the two disagree, the engine manufacturer’s instruction is the one that keeps the warranty and the piston intact.