Fan Selection Calculator

Because Brake Horsepower is a mechanical/aerodynamic quantity -- the shaft power the fan wheel itself needs, computed purely from Airflow Rate, Static Pressure, and Fan Efficiency. Motor Efficiency only describes how much ELECTRICAL power the motor draws to deliver that same shaft power, so it feeds into Electrical Input Power and Annual Energy Use, not into how big a motor you need to select in the first place.

Because motors are manufactured in a fixed set of standard horsepower ratings, not arbitrary fractional sizes -- if your computed Brake Horsepower is 2.2 HP, there's no 2.2 HP motor on a supplier's shelf, so this calculator selects the next standard size at or above your requirement (3 HP in that example). The ladder is NEMA MG 1's general-purpose list, which ends at 500 HP; a high-pressure industrial duty that computes above 500 HP is rounded up to the next whole 100 HP, because motors that large are custom large machines rather than catalog stock.

Because neither can be derived from the four aerodynamic inputs here. Tip speed is pi × wheel diameter × shaft RPM, and this calculator asks for neither -- a 5,000 CFM duty can be met by a slow large wheel or a fast small one, at completely different tip speeds. Sound level needs more still: the fan type, the blade-pass frequency, and the manufacturer's AMCA-certified sound-test data, plus a room or distance correction to turn sound power into the sound pressure a person actually hears. Rather than print a number built on assumed values, this calculator leaves both out -- use the fan manufacturer's published curve and sound data for those.

It goes down -- Fan Efficiency sits in the denominator of the Brake Horsepower formula, so a more efficient fan wheel (moving from a forward-curved design toward a backward-curved or airfoil design, for example) needs less shaft power to move the same Airflow Rate against the same Static Pressure, which can also let you select a smaller, cheaper motor.