How do you size a fan motor from CFM and static pressure?

Multiply airflow (CFM) by static pressure (in. w.c.) by a power conversion basis, then apply a service-factor margin. At 12,000 CFM, 6 in. w.c., 0.00022, and a 1.15 margin you get about 18.2 hp.

What is the power conversion basis in fan sizing?

It is the horsepower needed per unit of CFM times inches of water column, and it embeds the assumed combined fan and drive efficiency. A value around 0.00022 reflects a reasonably efficient fan; a less efficient unit uses a higher number.

Why apply a service factor or sizing margin?

Real systems see filter loading, damper changes, and air-density swings that raise brake horsepower. A 1.15 margin gives roughly 15 percent headroom so the motor is not running at its limit at the worst operating point.

Is brake horsepower the same as motor horsepower?

No. Brake horsepower is what the fan shaft actually absorbs at the duty point; motor horsepower is the next standard frame size up with margin. This tool estimates the sized motor, but you still confirm against the fan curve's BHP.

Why is my calculated motor size between standard frames?

The 18.2 hp result is an estimate, not a catalog frame. You round up to the nearest standard motor, typically 20 hp here, and verify it covers BHP plus temperature rise at the actual duty point.

What happens if I oversize the fan motor?

An oversized motor runs at low load with poor power factor and efficiency, and a fan curve pushed to handle the larger motor can move more air than needed, wasting energy. Right-sizing with a modest margin is better than padding heavily.

Industrial Fans, Blowers & Air Movement Equipment calculator

Motor Sizing Calculator

Fan and blower motor sizing starts from the air the system must move and the resistance it must overcome. This calculator estimates required motor horsepower from design airflow, static pressure, a power conversion basis, and a service factor or sizing margin, giving you a fast first-pass number before you open a fan curve. Mechanical engineers, HVAC designers, and equipment specifiers use it to scope a motor early, sanity-check a vendor selection, and avoid the twin failures of an overloaded motor or a wastefully oversized one. It is a screening tool: the real selection still has to land at the fan's duty point.

What this calculator does

Estimate fan motor sizing from airflow, static pressure, a power conversion basis, and service factor.
Use it when selecting a motor size for centrifugal fans, axial fans, blowers, or exhaust systems before checking final brake horsepower.
It estimates fan or blower motor horsepower by combining design airflow, static pressure, a power-per-CFM-inch conversion factor, and a service or margin multiplier.

Formula used

Estimated motor size = design airflow × design static pressure × power conversion basis × motor service factor or sizing margin
Confirm final motor selection against brake horsepower, service factor, temperature rise, and fan curve duty point.

Inputs explained

Design airflow: Use the airflow required at the fan inlet or outlet for the selected operating point.
Design static pressure: Use external static pressure or total system pressure basis consistently with your sizing method.
Power conversion basis: Use the conversion basis from your estimating standard, fan law model, or preliminary sizing worksheet.
Motor service factor or sizing margin: Apply the motor service factor, drive loss allowance, altitude allowance, or engineering sizing margin.

How to use the result

Use it for early scoping or to validate a quoted motor size before you have a full fan curve and brake horsepower data in hand.
The single conversion basis bakes in an assumed fan and drive efficiency, so it cannot capture how efficiency shifts along the fan curve and should never replace a manufacturer's brake horsepower selection.

Current U.S. benchmarks

Steel mill PPI stands at 348.53 (BLS, May 2026), up 6.7% from a year earlier. New factory orders are up 2.3% year over year (Census).

Common questions

How do you size a fan motor from CFM and static pressure? Multiply airflow (CFM) by static pressure (in. w.c.) by a power conversion basis, then apply a service-factor margin. At 12,000 CFM, 6 in. w.c., 0.00022, and a 1.15 margin you get about 18.2 hp.
What is the power conversion basis in fan sizing? It is the horsepower needed per unit of CFM times inches of water column, and it embeds the assumed combined fan and drive efficiency. A value around 0.00022 reflects a reasonably efficient fan; a less efficient unit uses a higher number.
Why apply a service factor or sizing margin? Real systems see filter loading, damper changes, and air-density swings that raise brake horsepower. A 1.15 margin gives roughly 15 percent headroom so the motor is not running at its limit at the worst operating point.
Is brake horsepower the same as motor horsepower? No. Brake horsepower is what the fan shaft actually absorbs at the duty point; motor horsepower is the next standard frame size up with margin. This tool estimates the sized motor, but you still confirm against the fan curve's BHP.
Why is my calculated motor size between standard frames? The 18.2 hp result is an estimate, not a catalog frame. You round up to the nearest standard motor, typically 20 hp here, and verify it covers BHP plus temperature rise at the actual duty point.

Last reviewed 2026-05-12.