Fan Performance Calculator
Calculates the motor power (HP) required to drive a fan based on airflow rate, static pressure, and combined fan and motor efficiencies. Used by HVAC engineers and system designers to select appropriately sized fan motors.
About this calculator
Fan shaft power is determined by the air power — the product of airflow and pressure — divided by the fan's mechanical efficiency. The required motor input power (in horsepower) is: P = (Q × SP) / (6356 × η_fan/100 × η_motor/100), where Q is airflow in CFM (cubic feet per minute), SP is static pressure in inches of water column (in. H₂O), η_fan is fan efficiency (%), and η_motor is motor efficiency (%). The constant 6356 converts the product of CFM and in. H₂O into horsepower. Air power (the theoretical minimum) equals Q × SP / 6356. Real systems require more power because both the fan wheel and motor are less than 100% efficient, and these two efficiencies multiply together. Typical fan efficiencies range 60–85% and motor efficiencies 85–97%.
How to use
An HVAC system moves 10,000 CFM against 2.5 in. H₂O of static pressure. The fan efficiency is 75% and the motor efficiency is 92%. Step 1 — air power: 10,000 × 2.5 / 6356 = 3.933 HP. Step 2 — divide by combined efficiency: P = 3.933 / (0.75 × 0.92) = 3.933 / 0.69 ≈ 5.70 HP. You would therefore select a 7.5 HP motor (the next standard size up) to ensure sufficient capacity and avoid overloading. This also provides headroom for motor service factor and system pressure variations.
Frequently asked questions
What does static pressure mean in fan performance calculations?
Static pressure is the resistance the fan must overcome to move air through the ductwork, filters, coils, and fittings in a ventilation system. It is measured in inches of water column (in. H₂O) and represents potential energy per unit volume of air. Higher static pressure means the system is more restrictive, requiring more fan power for the same airflow. System designers calculate total static pressure by summing the pressure drops across every component in the air path, then use this value to select a fan that can deliver the required CFM at that resistance.
How does fan efficiency differ from motor efficiency in a fan system?
Fan efficiency (also called total or static efficiency) describes how well the fan wheel converts mechanical shaft power into air power — it accounts for aerodynamic losses in the impeller and housing. Motor efficiency describes how well the electric motor converts electrical input power to mechanical shaft power, accounting for copper, iron, and friction losses. Both efficiencies multiply together to give the system's overall wire-to-air efficiency. A fan with 75% fan efficiency and a 90% efficient motor results in an overall system efficiency of only 67.5%, meaning nearly a third of electrical input is lost as heat.
Why is the constant 6356 used in the fan power formula?
The constant 6356 is a unit-conversion factor that makes the fan power formula consistent when airflow is in CFM and pressure is in inches of water column, yielding power directly in horsepower. It is derived from the equivalences: 1 HP = 33,000 ft·lbf/min, 1 in. H₂O = 5.192 lbf/ft², and converting CFM (ft³/min) × lbf/ft² gives ft·lbf/min. Dividing by 33,000 and simplifying produces the 6356 constant. If you use SI units (m³/s and Pascals), the formula becomes P (watts) = Q × ΔP / (η_fan × η_motor), with no additional conversion constant needed.