HVAC Ductwork, Air Handling & Mechanical Products worked example

Fan Motor Sizing at 49% fan total efficiency: a worked example

Suppose fan total efficiency falls to 49%. This page works the full calculation at that level so you can see exactly which result moves and by how much. Estimate the required fan motor shaft power or brake horsepower from design airflow, total static pressure, fan efficiency, and a motor service factor. Use for supply, return, and exhaust fan selection in air handling units and ductwork systems.

The inputs for this scenario

  • Design airflow: 10,000 CFM (held at the documented default)
  • Total static pressure: 2.5 in. w.g. (held at the documented default)
  • Fan total efficiency: 49 % (the input this scenario stresses; the baseline uses 68)
  • Motor service factor: 1.15 x (held at the documented default)

Working through the calculation

  • The calculation starts from the formula this tool documents: Fan shaft power (BHP) = (CFM × total static pressure) ÷ (6356 × fan efficiency) × service factor.
  • Result works out to 1,408,750 units at these inputs, and this is the headline figure for the scenario.
  • Base product works out to 1,225,000 value at these inputs.
  • Multiplier works out to 1.15 x at these inputs.
  • Factor A x B works out to 25,000 value at these inputs.

How this compares with the baseline

  • Against the tool's baseline example, where fan total efficiency sits at 68% and the headline result is 1,955,000 units, this scenario comes in 27.94% below the baseline at 1,408,750 units.
  • It computes fan shaft brake horsepower from airflow, total static pressure and fan efficiency, then scales it by the motor service factor to indicate the standard motor size to select. When the numbers land here, the stressed input is the lever to work; the walkthrough above shows exactly how much each output recovers as it climbs back toward the baseline.

Results at a glance

  • Result: 1,408,750 units (headline result)
  • Base product: 1,225,000 value
  • Multiplier: 1.15 x
  • Factor A x B: 25,000 value

Run it with your numbers

  • To rerun this with your own numbers, open the live Fan Motor Sizing calculator, set fan total efficiency to your actual value, and adjust the remaining inputs to match your operation.

Last reviewed 2026-05-12.