SFP for a given fan system and operating point (combination of flow rate and pressure rise) is defined as:
where:
*  is the electrical power used by the fan (or sum of all fans in the ventilation system)  [kW]
*  is the gross amount of air circulated through the fan (or ventilation system)  [m/s]
There are various sub-definitions of SFP for different specific applications, including SFP (building energy performance calculations), SFP (for performance verification tests), SFP (individual fan), SFP (air handling unit), SFP (fan coil unit), and SFP (whole building). These are explained in  and in part in. Reference 1 also describes how account for intermittently operated fans, e.g. kitchen hoods, and part-load performance in variable air volume (VAV) systems.
SFP can be expressed in the following equivalent SI units: