Octane Blend Calculator

The formula

blended octane = (octaneA × volumeA + octaneB × volumeB) ÷ (volumeA + volumeB)

A plain volume-weighted average. Twenty litres of 95 on top of twenty litres of 102 gives 98.5, not 102 — the half-tank of 95 does not disappear because you put good fuel on top of it. Rearranged for a target, the volume of the higher-octane fuel you need is:

volumeB = volumeA × (target − octaneA) ÷ (octaneB − target)

That second form is the one that stops people wasting money. Chasing the last point of octane costs disproportionately: lifting 20 litres of 95 to 98 takes 15 litres of 102, and pushing that same blend one more point to 99 takes another 12.

Blending is close to linear — but not exactly

Octane numbers are a measured behaviour, not a physical quantity like mass, so they do not have to add up. In practice, blending two ordinary petrol grades by volume is accurate to roughly half a point, which is finer than any pump will meter for you.

The exception is anything with a lot of ethanol or oxygenates. Ethanol's blending octane is famously non-linear: it contributes far more in the first few percent than it does at the top end, so a 10% ethanol splash raises the number more than its share of the tank suggests. If the fuel you are adding is E85, use the E85 mix calculator instead — the ethanol percentage, not the octane number, is what your tune cares about.

RON, MON and AKI — the same fuel, three numbers

Before blending anything, make sure both numbers are on the same scale.

Because MON is typically 8–10 points below RON, AKI lands about 4–5 points below RON for the same fuel. That is the whole reason US 91 and European 95 are roughly the same product, and why a European car rated for 95 RON is not being short-changed on American 91 AKI. Mixing a RON figure with an AKI figure in this calculator will give you a number that means nothing.

The glossary entry for AKI and the RON/MON entry spell out the test conditions if you want the detail.

More octane is not more power

Octane measures resistance to knocking, not energy content. In fact high-octane pump fuel often carries marginally less energy per litre than regular. Putting 100 RON in an engine that was not knocking on 95 gains nothing at all.

What octane buys is headroom. A modern ECU pulls ignition timing when the knock sensor hears detonation; better fuel lets it keep the timing, and that is where the power comes from. So the gain is real on an engine that is knock-limited — high compression, hot intake air, boost, a track session, a hot day — and zero on one that is not.

If you are running boost, the relevant number is effective compression ratio, which is what actually decides whether your static ratio and your boost pressure need more fuel than the pump sells.

Practical cautions

• Leaded race fuel destroys catalytic converters and oxygen sensors, usually within a tank. It is also illegal on the road almost everywhere. If a fuel is sold "for off-road/competition use only", that is what it means.
• Check the ethanol content, not just the octane. Adding a high-ethanol fuel changes the air-fuel requirement — E85 needs roughly 30% more fuel volume — and a stock fuel system may not have the injector or pump headroom. The injector calculator will tell you whether yours does.
• Blend in the tank, not in a can. Fuel stratifies less than people fear, but a partial fill followed by a drive mixes far better than pouring one fuel on top of another in a jerry can.
• An old tank of fuel does not stay at its octane. Petrol loses light fractions over months; the octane falls with them. A blend built on eight-month-old 95 is not starting where you think it is.

FAQ