A fan must overcome resistance when pushing air from the discharge of the fan through the duct to some final outlet. This resistance is known as static pressure. Static pressure in a typical duct run is caused by the velocity of the air, type of duct material, elbows/fittings, dampers, diffusers, etc.
Fan static pressure is measured in inches of water gauge
To calculate static pressure, you must the ductwork system configuration. See link for an example on how to calculate static pressure.