How to Use the Fan Power Calculator
Sizing the motor for an HVAC or exhaust fan starts with calculating the brake horsepower (BHP) required for a given airflow and static pressure. This calculator takes your airflow (CFM), static pressure (in. wg), and fan efficiency, and returns the required brake horsepower.
The formula is BHP = (CFM × static pressure) / (6356 × efficiency), the standard AMCA-style equation used throughout US HVAC design. Static pressure represents the total resistance of the duct system — ductwork, filters, hoods, and fittings combined — and efficiency depends on fan type (centrifugal, axial, etc.) and the specific model's performance curve.
The result is fan shaft power. When selecting an actual motor, account for belt-drive losses and a service factor, typically rounding up to the next standard motor size above the calculated value. For detailed system design, consult the fan manufacturer's performance curve and a qualified mechanical or electrical engineer.
Frequently Asked Questions
Static pressure is the total resistance of the duct system — ductwork, filters, hoods, and fittings combined — typically taken from your duct calculation or system design documents.
Centrifugal fans commonly run 50-70% efficient, and axial fans 60-80%, but the exact figure should come from the manufacturer's fan performance curve for the specific model.