Octane number is defined as the percentage by volume of iso-octane (2, 2, 4-trimethylpentane) in a mixture on n-heptane and iso-octane. Octane number is a measure resistance of gasoline (petrol) to knocking in spark ignition internal combustion engines.

Example-If a gasoline contains 80% of iso-octane and 20% of n-heptane then the octane rating of the fuel is 80.

The engines having high compression ratio require high octane rating fuel to resist the knocking.