This document defines the calculation method for determining the fan energy index (FEI), which is an energy efficiency metric for fan duty points. This metric provides a standardized and consistent basis to compare fan energy performance across fan types and sizes at a given fan duty point. This document is applicable to fans driven by motors and fans without drives. It is not applicable to circulating fans or air curtains. The fan energy index can only be calculated for fan duty points above a minimum air power of 125 W (where air power is the product of volume flow rate and fan static pressure) or above a minimum volume flow rate of 2.0 m3/s.