Octane Rating Converter

Two scales, one numeral

An American in Europe sees “95” on the pump and assumes the fuel is far better than the “87” at home. A European in America sees “87” and assumes the opposite. Both are reading the wrong axis.

Every petrol has two octane numbers, from two different tests. North America, Canada and Brazil post the average of them, called the anti-knock index and often printed on the pump as (R+M)/2. Europe, the UK, India, Japan and Australia post the higher of the two, the Research Octane Number, on its own.

Three things fall straight out of that table. A US regular and an Indian or Australian regular are the same fuel wearing different numbers. European regular is genuinely a step above them — about four RON — which is a real difference and not the eight-point one the pumps suggest. And US premium and UK super are, by this arithmetic, the same petrol.

The practical rule is short: never compare a pump number across a border without knowing which scale each is on. The numeral alone carries no information about the fuel.

Why there is no exact conversion

AKI is the average of RON and MON, so converting between AKI and RON needs the gap between the two. That gap is called sensitivity, and it is a property of the particular fuel — not a constant, and not printed anywhere.

For ordinary pump petrol it runs about 8 to 10. Highly aromatic fuels and ethanol blends sit higher, sometimes 12. That is enough to move a converted figure by two and a half numbers, which is more than the difference between two adjacent grades on a US forecourt.

This is why the calculator shows a range. Any tool that gives a single number for “87 AKI in RON” has chosen a sensitivity on your behalf and not said so — which is fine as an approximation and misleading as an answer.

The direction matters as much as the size. Converting up from AKI to RON, a higher sensitivity gives a higher number. Converting down from RON to AKI, a higher sensitivity gives a lower one. The two move opposite ways because AKI sits at the midpoint, and getting that backwards produces an answer that looks reasonable and is wrong by the full width of the band.

One case is exact and worth knowing: if a fuel’s data sheet gives both RON and MON — as a racing fuel or an aviation gasoline spec will — then AKI is their average with nothing assumed at all, and sensitivity is simply their difference.

Octane is not energy

The most persistent misconception about petrol grades is that a higher octane number means more energy, more power, or a cleaner burn. It means none of those. It measures one thing: how much compression the fuel tolerates before it detonates rather than burning smoothly.

The clearest evidence is what happens when ethanol is blended in. Ethanol is a powerful octane booster and carries roughly a third less energy per litre than petrol, so the two properties move in opposite directions.

So the higher-octane fuel in that table is the one with less energy in it, and by a margin drivers can measure at the pump. It is a large part of why fuel economy fell slightly when E10 replaced E5 in several markets, and why a flex-fuel car on E85 uses noticeably more fuel per mile despite the fuel’s much higher octane rating.

What follows for an ordinary car is simple and worth stating plainly: buying a higher grade than the handbook requires gains nothing. An engine built for 91 RON cannot exploit knock resistance it was not designed around; the ignition timing is already as advanced as its knock sensor allows. The exception is an engine whose manufacturer specifies a higher grade — there the extra octane is being used, and dropping below the stated minimum makes the engine retard its timing, costing power and economy before it costs anything worse.

What the tests actually do

Both octane numbers come from the same apparatus: a single-cylinder engine with a compression ratio that can be changed while it runs. The fuel is tested against blends of two reference compounds, and the octane number is the percentage of iso-octane in the blend that knocks at the same compression.

That is why the scale runs to 100 and can exceed it. Pure iso-octane is 100 by definition and pure heptane is 0; a fuel more knock-resistant than iso-octane rates above 100, which is how Japanese high-octane petrol is sold at RON 100 and why aviation gasoline carries numbers above it.

The MON conditions are deliberately harsher — hotter, faster, with timing that pushes the fuel — so the same petrol always rates lower on it. Sensitivity, the gap between the two, therefore has a physical meaning: it says how much a fuel’s knock resistance falls off as conditions get severe.

That is also the argument for each posting convention. RON alone describes part-throttle, moderate-load running, which is most driving. The average acknowledges that a hot, hard-worked engine sees something closer to the MON condition. Neither is wrong; they are different summaries of the same pair of measurements, which is exactly why the numbers cannot be compared as printed.

Choosing a grade abroad

The question this page usually gets asked is practical: I am driving a car in another country, which pump do I use?

Start from the handbook, not from the pump. The manufacturer states a minimum, and it is stated on the scale used where the car was sold. A European car specifying “95 RON minimum” needs about 91 AKI, which is US premium on the west coast — not regular. An American car specifying “87 AKI” needs about 91 RON, which is comfortably below European regular, so anything on a European forecourt is fine.

Round up, not down. Because the conversion is a band, the safe reading is the pessimistic end of it. Going one grade higher than the arithmetic suggests costs a little money; going one lower risks knock, and a modern engine responds by retarding timing, which quietly costs power and economy.

Watch for the local oddities. Some US mountain states have historically sold 85 AKI regular, on the reasoning that thinner air reduces knock tendency; those grades are being withdrawn but still appear. Japanese regular at RON 90 is lower than the European figure. And ethanol content varies by country and season within the same posted grade, which changes both the sensitivity and the energy content.

Do not read the number as quality. Detergent packages, additives and cleanliness are separate matters from the octane rating, and a higher number tells you nothing about them. If a car runs contentedly on the minimum grade, a higher one is buying knock resistance the engine will not use.

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Sources and methodology

The two octane numbers come from two separate ASTM test methods run on the same standardised variable-compression engine under different conditions, which is why one fuel has two ratings and why the gap between them is a property of the fuel rather than an error. European pump grades are specified as RON under EN 228; North American ones are posted as the average of the two, usually printed on the pump as (R+M)/2.

• ASTM D2699 — Standard Test Method for Research Octane Number of Spark-Ignition Engine FuelASTM International · verified 2026-09-07 · The Research Octane Number test conditions — 600 rpm and a 52 °C intake — which are the gentler of the two and produce the higher number
• ASTM D2700 — Standard Test Method for Motor Octane Number of Spark-Ignition Engine FuelASTM International · verified 2026-09-07 · The Motor Octane Number test conditions — 900 rpm, a 149 °C intake and variable ignition timing — which are harsher, so MON is always below RON for the same fuel and the gap between them is sensitivity
• EN 228 — Automotive fuels: unleaded petrol requirements and test methodsEuropean Committee for Standardization · verified 2026-09-07 · That European pump grades are specified and posted as RON with a separate minimum MON, which is why a European pump number is not comparable with a North American one as printed