Multiply the fan's efficiency by the motor's efficiency. If the fan has an efficiency of 0.9, and the motor has an efficiency of 0.75, then 0.9 x 0.75 = 0.675.
Multiply your answer by the drive efficiency. If it has an efficiency of 0.92, then 0.675 x 0.92 = 0.621.
Multiply your answer by 6,345, a constant that relates the different airflow factors. With this example, 0.621 x 6,345 = 3,940.
Multiply your answer by the fan's power input, measured in brake horsepower (bhp). If the power input is 0.5 bhp, then 3,940 x 0.5 = 1,970.
Divide your answer by the system airflow in cubic feet per minute. It the fan moves 200 cfm, then 1,970 / 200 = 9.85. This is the exhaust fan's static pressure.