Pressure loss to air flow resistance, and caused by molecular friction and surface friction, such as air passing through a system. It is measured in Pascals (Newtons/square meter) or inches water level and with the square of the air velocity. Thus, doubling the flow rate through a given system has the effect of four times the pressure, and it halves the pressure by three-quarters.

It is difficult to estimate the pressure loss in devices with complicated air passages, so experience with similar systems is required. For pipelines, graphs of pressure and loss factors are the subject of a thorough study of each component in the system.

The static pressure exerts the pressure in the duct wall and corresponds to the potential energy. Velocity pressure is flow pressure and corresponds to kinetic energy. The sum of the two is the total pressure and corresponds to the increase in the total energy provided by the fan.

It is estimated by calculating the velocity pressure of each component passing through the piping system. It multiplies the component with a constant, called a factor k, to provide the estimated static pressure loss of the component. The static pressure of the system is then the sum of the static pressures of all components, and the velocity pressure is calculated from the exhaust air velocity.