It is defined as the percentage by volume of normal-cetane in a mixture of normal-cetane and alpha-methyl naphthalene. It is used for diesel fuels. Normal-cetane is assigned a cetane number of 100 and alpha-methyl naphthalene is assigned cetane number of zero.

Cetane number gives the time period between the start of injection and ignition of fuel. Higher the cetane number, shorter will be the time period between the start of injection and ignition of fuel.