Octane number of a petrol sample is defined as the % of iso-octane in the mixture of iso-octane & n-heptane, which has similar,knocking to the petrol sample.

Cetane number of diesel sample is defined as % of n hexadecane,in the mixture of n-hexadecane & 2-methyl naphthalene which,has same ignition character like the ignition character of the diesel under test.