HVAC Ductwork, Air Handling & Mechanical Products worked example

Fan Motor Sizing at 78% fan total efficiency: a worked example

This scenario runs the fan motor sizing calculation on the strong side: 78% fan total efficiency, with every other input held at its documented default. Use this when selecting a supply fan, return fan, or exhaust fan motor for an air handling unit or ductwork system. Enter the design CFM, total static pressure in inches water column, fan total efficiency, and a service factor to get estimated brake horsepower. Round up to the next available motor frame size and confirm with the fan curve.

The inputs for this scenario

  • Design airflow: 10,000 CFM (unchanged)
  • Total static pressure: 2.5 in. w.g. (unchanged)
  • Fan total efficiency: 78 % (raised for this scenario; the documented default is 68)
  • Motor service factor: 1.15 x (unchanged)

Working through the calculation

  • Applying the documented formula (Fan shaft power (BHP) = (CFM × total static pressure) ÷ (6356 × fan efficiency) × service factor) to the inputs above produces each figure below.
  • At this operating point the engine returns 2,242,500 units for result, the number this scenario is built around.
  • At this operating point the engine returns 1,950,000 value for base product.
  • At this operating point the engine returns 1.15 x for multiplier.
  • At this operating point the engine returns 25,000 value for factor a x b.

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 14.71% above the baseline at 2,242,500 units.
  • Use it when selecting or replacing an AHU or inline fan motor, validating a vendor selection, or diagnosing an overloaded fan after duct changes raised static pressure. Treat this as a target state: the delta against the baseline quantifies what the improvement is worth before you commit to chasing it.

Results at a glance

  • Result: 2,242,500 units (headline result)
  • Base product: 1,950,000 value
  • Multiplier: 1.15 x
  • Factor A x B: 25,000 value

Run it with your numbers

  • Every input above is editable in the live Fan Motor Sizing calculator, which recalculates instantly and can be shared with the inputs intact.

Last reviewed 2026-05-12.