Octane Rating Converter - RON, AKI, MON

Gasoline octane ratings measure a fuel’s ability to resist “knocking” or pre-ignition inside an engine.
The same fuel carries different numbers depending on which rating system is used, which is why a European driver renting a car in Texas thinks the fuel has suddenly got worse. It has not. The number on the pump is measured differently.

RON: Research Octane Number Used in Europe, Australia, Asia, and most of the world. Measured under low-speed, low-load engine conditions. Regular unleaded: 95 RON in Europe, 91–92 RON in Australia and Japan. Premium: 98 RON. Super premium: 100+ RON.

AKI: Anti-Knock Index (Pump Octane) Used in the United States and Canada. Shown on yellow stickers at fuel pumps as (R+M)/2. Formula: AKI = (RON + MON) / 2 Using the common approximation MON ≈ RON − 9: AKI ≈ RON − 4.5 Regular US gas: 87 AKI ≈ 91.5 RON. Premium: 91–93 AKI ≈ 95.5–97.5 RON.

MON: Motor Octane Number Measured under high-speed, high-load engine conditions. Always lower than RON, typically by 8 to 10 points. MON ≈ RON − 9 (common approximation) Less visible to consumers but used in the AKI calculation.

Sensitivity: the gap this page has to guess at. The difference RON − MON has a name, fuel sensitivity, and it is a real property of the blend rather than a constant. Straight-run hydrocarbons sit near 7; heavily aromatic and ethanol-blended pump fuels run 10 to 12. Modern turbocharged engines actually prefer a high-sensitivity fuel, because MON is measured under conditions harsher than anything a road engine sees.
That is why this converter can only ever be an approximation. It assumes a sensitivity of 9, which is the middle of the range and close to typical summer pump gasoline. If your fuel’s real sensitivity is 11, a 95 RON fuel has a MON of 84 and an AKI of 89.5, not the 90.5 shown here. One point of AKI, and worth knowing about if you are chasing knock on a tuned engine.

Common fuel grade equivalents:

Note that every row satisfies AKI = (RON + MON) / 2, which is worth checking on any octane chart before you trust it.

What about E10? Ten percent ethanol is now standard in most US regular gasoline, so the 87 AKI row already is E10 in practice. Ethanol has a very high blending octane, which is part of how refiners hit 87 cheaply, and it also raises sensitivity: an E10 fuel usually has a wider RON to MON gap than the straight hydrocarbon it replaced. E85 is a different animal entirely, roughly 100 to 105 AKI, and is not comparable on this scale because it carries about 27% less energy per gallon.

A note on what octane is not. A higher number does not mean more power or better fuel economy. It means the fuel resists detonating before the spark plug asks it to. Putting 93 in an engine designed for 87 buys you nothing except a more expensive tank. The gain only appears in engines whose knock sensors are holding timing back, which is most turbocharged cars and very few naturally aspirated ones.

Note: These conversions use the standard approximation (MON = RON − 9). Actual MON values vary by fuel blend and season. Always follow your vehicle manufacturer’s recommendation.