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what is the percentage octane in petrol?

Virtually all petrol stations offer a variety of fuels with different Octane ratings, and the numbers usually range between the high 80s and the mid 90s. This number is the resistance of the fuel to self-igniting. Self ignition of fuel is bad because it may burn before the piston is in the correct position to benefit from the explosion of the fuel, which would reduce the efficiency of the engine. It is this reason - the timing of the ignition of the fuel - that makes higher Octane petrol a better fuel than lower rated petrol. It has nothing to do with the heat energy stored in the fuel, the products of the reaction, or anything else.

This number is based on the behaviour of a mixture of Octane and Heptane which is the other major component of standard fuel. For example, a mixture of 90% Octane and 10% Heptane has a standard resistance to spontaneously bursting into flame. That resistance is given the value "90". If the amount of Octane is increased to 95% and the Heptane reduced to 5%, the resistance of the fuel is rated as "95".

This effectively means that the rating of petrol is a good guide to the percentage of Octane in it. However this is not always the case. Fuels may contain other chemicals but as long as the overall mixture has the same resistance to self igniting as, say, the 90/10 mix of Octane/Heptane, that fuel can be labeled "90" Octane rating.