What is the difference between RON, MON and AKI?

RON (Research Octane Number) is measured under mild test conditions and is the figure shown on European and most Asian pumps. MON (Motor Octane Number) uses harsher conditions and is a few points lower. AKI (Anti-Knock Index), shown in the US and Canada, is the average of the two: (RON + MON) / 2.

Why is US 87 the same as European 91-92?

Because the US displays AKI (the RON/MON average) while Europe displays RON. US regular at 87 AKI is roughly 91-92 RON, mid-grade 89 AKI ≈ 93-94 RON, and premium 91-93 AKI ≈ 95-98 RON. The fuel is comparable; only the displayed scale differs.

Does higher octane give more power?

Only if your engine needs it. Octane measures resistance to knock (pre-ignition), not energy content. Engines tuned for high octane make more power and efficiency on it; in a low-compression engine, premium fuel gives no benefit. Always meet, but rarely exceed, the manufacturer's minimum.

What happens if I use lower octane than required?

The engine may knock — uncontrolled detonation that can damage pistons and valves. Modern engines retard ignition timing to prevent knock, which reduces power and economy. Persistently under-fuelling a high-compression engine risks long-term damage, so meet the specified minimum.

Is this conversion done online?

No. The octane scale conversion and country data run entirely in your browser. Nothing is sent anywhere.

What is the Fuel Octane Rating Reference?

Reference table comparing RON, MON and AKI octane ratings for petrol grades by country, explaining how the three scales relate and which pump number each country displays. It runs free in your browser on Gera Tools, with nothing uploaded.

Fuel Octane Rating Reference

Name: Fuel Octane Rating Reference
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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Petrol octane ratings

Octane rating measures a fuel’s resistance to knock — the higher the number, the more the fuel resists pre-ignition under compression. But three different scales exist, and a country’s pump number depends on which it displays. This reference compares RON, MON and AKI, converts between them, and lists what each grade means by country.

How it works

The same fuel is tested two ways. RON uses mild conditions; MON uses harsher, higher-load conditions and reads several points lower. Europe, the UK, Australia and most of Asia display RON. The US and Canada display AKI, the Anti-Knock Index, defined as AKI = (RON + MON) / 2. The gap between RON and MON (the “sensitivity”) is typically 8-10 points for pump petrol, so a fair estimate is MON ≈ RON − 9 and therefore AKI ≈ RON − 4.5. The converter applies these relationships so you can translate any pump number to the others.

Country-by-country grade guide

Understanding what the pump number means in each country prevents confusion when travelling or importing a vehicle:

Country / Region	Scale shown	Regular	Mid	Premium
USA and Canada	AKI	87	89	91–93
UK and most of Europe	RON	95	—	97–99
Australia	RON	91	95	98
Japan	RON	89	—	96
Brazil	RON	87 (ethanol blend)	—	95
India	RON	87–91 (varies by city)	—	95

US “87” and European “95” are not far apart in practice: 87 AKI ≈ 91-92 RON. A European car specifying 95 RON minimum needs approximately 91 AKI (US premium) — not 87 regular.

What knock actually is and why it matters

Knock — also called detonation or pinging — happens when fuel-air mixture auto-ignites before the spark plug fires. Instead of a smooth, controlled burn that pushes the piston down, you get an uncontrolled pressure spike that hammers the piston and can damage rings, pistons, and the cylinder head over time.

Modern engines use knock sensors to detect this and retard ignition timing automatically. That prevents immediate damage but costs power and fuel economy. The engine is running a defensive strategy, not an optimal one. Persistent knock in an older engine without sensors causes real long-term harm.

Tips and examples

Match the minimum, not the maximum. Your owner’s manual specifies a minimum octane. Meeting it is fine; exceeding it offers no benefit in most naturally aspirated engines.
Turbo and high-compression engines are more sensitive. Many specify premium as required rather than recommended — that means the engine timing strategy assumes premium; running regular will measurably reduce performance and efficiency.
Fuel ethanol content affects the effective octane. In some regions, pump blends contain 10–15% ethanol (E10/E15), which naturally raises octane slightly. US E10 regular at 87 AKI contains that ethanol boost, while pure hydrocarbon 87 would behave differently.
The conversion is an estimate. The RON-to-AKI relationship assumes a typical sensitivity of about 9 points. Reformulated, oxygenated, or premium synthetic blends can have different sensitivities, so the conversion is a guide rather than a guarantee.