Octane rating is a relative quantity specification of the engine fuel antiknock rating to detonation of engine fuels used in gasoline internal combustion engines. Engine fuel can be compared to an isooctane mix, the conditional octane rating of which is 100, and n-heptane with a conditional rating of 0. The percentage (in terms of volume) of isooctane in the mixture equivalent to the tested fuel in terms of antiknock rating at standard test conditions is the fuel octane rating.

The hydrocarbons octane ratings for natural gas octane rating calculation by engine fuel are as follows:

• methane – 110;
• ethane – 108;
• propane – 105;
• n-butane and i-butane – 94;
• n-pentane and isopentane – 70.