RON and AKI: why octane numbers differ by country
RON and AKI are two ways of measuring how well petrol resists engine knock. RON is used in most of the world, AKI in the US and Canada. They test the same property with different methods, so the numbers aren't directly comparable without a conversion.
In this entry
Two labels for the same idea
RON and AKI are two ways of writing the same idea: how well petrol resists knock, an uncontrolled ignition inside the engine that can sound like pinging or rattling. The higher the number, the more the fuel resists it.
They are not two different properties of the fuel. They are two different tests, run on the same fuel and give two different numbers. A car built for one region's labelling is not being sold a lesser fuel when it crosses into a country that uses the other scale.
How RON and MON are measured, and where AKI comes from
Research Octane Number (RON) is measured in a laboratory engine run under mild conditions: low speed, low temperature. Motor Octane Number (MON) uses the same kind of engine but pushed harder, at higher speed and with hotter intake air, which is closer to how an engine behaves under load.
MON numbers come out lower than RON for the same fuel, usually by about 8 to 10 points. The Anti-Knock Index, AKI, is simply the average of the two.
Which countries use which scale
Which scale a country prints on its pumps is a matter of regulation and history, not the fuel itself.
Where each scale is used
Matching a RON grade to an AKI grade
Because AKI is pulled down by the lower MON figure, a US number reads roughly 4 to 5 points lower on the AKI scale than on RON. The matches below are approximate, since exact figures vary by fuel batch and how each country runs its test.
Approximate matches. RON figures on the left, the AKI figures commonly sold under them.
What this means if you drive across regions
Roughly, yes, but treat it as a rough guide rather than a conversion formula. 91 AKI sits close to 95 to 96 RON, and the two fuels perform almost the same in an engine tuned for either. The bigger practical question is always what your vehicle's manufacturer specifies, not which scale the number happens to be written in.
A car imported from a RON country into an AKI country, or the other way around, should use the pump grade closest to what its manual recommends, converting with the approximate matches above rather than by number alone.
How this shows up on FuelTheGuide
We label every price with the scale that country actually publishes: RON grades such as 92, 95, 98 and 100 for most of the world, and AKI grades such as Regular 87, Midgrade 89 and Premium in the low 90s for the US and Canada.
For cross-country comparisons, converting every number into one scale would sacrifice data the site doesn't have. Instead we group grades into standard and premium petrol, using each country's own everyday grade as the standard, so you're comparing like roles rather than exact octane figures.
Questions and answers
Why does the US use a different scale from most other countries?
There is no technical reason, only history. The AKI method, averaging RON and MON, became the US and Canadian standard decades ago, and pump labelling laws there still require it.
Is 91 AKI the same fuel as 95 RON?
Almost. As a rough guide, 91 AKI corresponds to about 95 to 96 RON, so they are close but not identical. Exact values vary by fuel batch and refinery.
Which scale is more accurate?
Neither is more accurate — they measure different things. RON uses milder test conditions that are closer to normal driving. MON uses harsher conditions. AKI splits the difference. All three are valid; they just give different numbers for the same fuel.
Do all countries that use RON test it the same way?
The test method is standardised internationally — ASTM D2699 for RON and ASTM D2700 for MON — so a 95 RON grade in Germany and a 95 RON grade in Australia are measured the same way. Local blending and ethanol content can still mean small differences in pump performance.
Sources and further reading
- Gasoline explained: octane in depth U.S. Energy Information Administration, eia.gov
- Motor fuels, general requirements (EN 228) European Committee for Standardisation, cenelec.eu
- Fuel prices and grade lists OpenVan.camp, open dataset, CC BY 4.0