Calculate fan horsepower from airflow in CFM or m³/h and pressure rise, using a typical 70% fan efficiency or your own, or solve for airflow or pressure.

Use static pressure with static efficiency, or total pressure with total efficiency, at one operating point.

70% is a typical mid-range value; most fans run about 50–85%. Use the fan curve value if you have it.

Motor and operating estimates
Default 100% assumes no drive loss; enter a lower value for a belt drive.

Default 1 adds no margin. This is a planning multiplier, not a NEMA service factor.

Blank omits electrical input and annual estimates.

Blank omits annual energy; needs motor efficiency.

Blank omits cost; needs operating hours and motor efficiency.

Related Calculators

Fan Horsepower Formula

Air Horsepower (ideal power delivered to the airstream):

AHP = (Q × pressure rise) / 6343.3115

where Q = airflow in CFM and pressure rise is in inches of water gauge (in. wg). The divisor matches the unit conversions used here; the legacy6356 convention gives slightly different results. SI air power is Q (m³/s) × pressure rise (Pa).

Brake Horsepower (power required at the fan shaft):

BHP = (Q × pressure rise) / (6343.3115 × ηf)

Use total pressure with total fan efficiency, or static pressure with static fan efficiency. Here ηf is a fraction from0 to1; enter efficiency as a percentage in the calculator. This is a low-pressure, effectively incompressible estimate.

Motor shaft-power planning estimate:

Motor shaft capacity estimate = (BHP / ηd) × planning multiplier

where ηd is mechanical drive efficiency. Electrical input = motor shaft power / motor electrical efficiency, including controller losses where applicable. A motor service factor describes permissible loading of rated mechanical output under specified conditions; it is not this planning multiplier.

Interpretation

Air power is the rate of work transferred to the air on the chosen pressure basis. Fan shaft power accounts for fan losses; motor shaft power additionally accounts for mechanical drive losses. Electrical input also includes motor/controller losses. Motor nameplate horsepower is mechanical output capacity, not electrical input horsepower. The planning estimate does not select or approve a motor.

Use manufacturer efficiency data at the operating point:

  • Propeller fans: use the tested efficiency for the chosen static or total pressure basis
  • Forward-curved centrifugal: do not assume peak efficiency at every flow
  • Backward-inclined / axial: confirm the fan type and efficiency basis
  • Airfoil centrifugal: use the manufacturer’s performance data

A fixed percentage tolerance does not establish a suitable fan or motor. Check the full manufacturer operating range, starting requirements, mechanical loading, duty and derating; do not routinely rely on loading above motor nameplate output.

Quick Reference Table

Approximate fan shaft horsepower at70% efficiency, using CFM and inches of water gauge with a matching efficiency basis:

  • 1,000 CFM @ 1 in. wg: ~0.23 BHP
  • 2,500 CFM @ 2 in. wg: ~1.13 BHP
  • 5,000 CFM @ 3 in. wg: ~3.38 BHP
  • 10,000 CFM @ 4 in. wg: ~9.01 BHP
  • 20,000 CFM @ 6 in. wg: ~27.03 BHP
  • 50,000 CFM @ 8 in. wg: ~90.08 BHP

Examples of motor mechanical-output ratings (hp): 0.25, 0.33, 0.5, 0.75, 1, 1.5, 2, 3, 5, 7.5, 10, 15, 20, 25, 30, 40, 50, 60, 75, 100. Availability and suitability depend on the motor, duty, starting and derating requirements; this calculator does not make a final selection.

FAQ

What units should pressure be in?
Select CFM, m³/s or m³/h and in. w.g., Pa or kPa in the calculator. It converts internally to SI using1 in. w.g. ≈249.08891 Pa and1 CFM =0.0004719474432 m³/s.

Should I use static pressure or total pressure?
Use total pressure with total efficiency, or fan static pressure with static efficiency, from the same operating point. Mixing the pressure and efficiency bases produces an inconsistent shaft-power estimate.

What fan efficiency should I use if I don’t know it?
Obtain it from the manufacturer’s data at your operating point and pressure basis. Efficiency varies with flow; this calculator supplies no generic fan-efficiency default.

Why is motor horsepower higher than brake horsepower?
Mechanical drive losses can require motor shaft output above fan shaft power. Electrical input additionally includes motor/controller losses. The optional planning multiplier applies to shaft capacity only; a nameplate service factor is a separate manufacturer rating and is not used to increase the operating-energy estimate.

Does air density matter?
Air density affects the fan operating point and performance data. The Q × pressure power relation uses actual volumetric flow and pressure at that point; its unit conversion constant does not assume an air density. Translating test data to field conditions or handling compressible flow requires appropriate engineering methods.