What is fan shaft power?

Shaft power (also called brake horsepower or BHP) is the mechanical power delivered to the fan shaft by the motor. It is always greater than the air power because real fans have losses from blade drag, recirculation, and bearing friction captured in the efficiency factor.

What efficiency value should I use?

Use the fan total efficiency from the manufacturer's performance curve at your operating point. Centrifugal fans typically peak at 70–85%; axial fans at 60–80%. If unknown, 70% (0.70) is a conservative default for preliminary sizing.

What is the difference between static and total pressure?

Static pressure is the potential energy component that overcomes duct resistance. Total pressure adds the velocity pressure (kinetic energy). For most duct system calculations, fan static pressure (FSP) is used in the power formula.

Why does the result differ from the motor nameplate power?

Shaft power is the power needed at the fan shaft. The motor must deliver more power to account for belt/drive losses (typically 2–5%) and must be derated for service factor. Always select a motor with a nameplate rating at least 10–15% above the calculated shaft power.

How do I convert CFM and inches w.g. to SI units?

1 CFM = 0.000471947 m³/s. 1 inch water gauge (in. w.g.) = 249.089 Pa. This calculator handles the conversion automatically when you select imperial units.

Fan Power Calculator

Name: Fan Power Calculator
Availability: InStock
Author: Nham Vu

Enter airflow rate, static pressure, and fan efficiency to calculate fan shaft power in kW or horsepower.

Fan Parameters

Unit System

Airflow Rate (m³/s)

Typical range: 0.1 – 50 m³/s

Fan Static Pressure (Pa)

Typical range: 50 – 5000 Pa

Fan Total Efficiency (%)

Typical range: 60 – 85%

Enter fan parameters and click Calculate

Summary

Enter airflow rate, static pressure, and fan efficiency to calculate fan shaft power in kW or horsepower.

How it works

Select your unit system: metric (m³/s, Pa, kW) or imperial (CFM, in. w.g., HP).
Enter the volumetric airflow rate produced by the fan.
Enter the fan static pressure rise across the fan.
Enter the fan total efficiency as a percentage (typically 60–85%).
The calculator applies P = (Q × ΔP) / η and displays shaft power in kW and HP.
Adjust any value and the result updates instantly.

Use cases

Size motors for HVAC air handling units.
Estimate energy consumption of industrial ventilation fans.
Verify fan performance data from manufacturers.
Compare fan efficiency at different operating points.
Calculate brake horsepower for centrifugal or axial fans.
Design ductwork systems by matching fan power to resistance.

Frequently Asked Questions

Last updated: 2026-06-10 · Reviewed by Nham Vu