Fan Calculator

Enter a fan's airflow rate, static pressure rise, and overall efficiency to calculate its air power and required shaft (brake) power using P = Q × ΔP.

Quick Facts

Air power formula
P_air = Q × ΔP
Flow rate (m³/s) times pressure rise (Pa) gives power in watts — the fundamental fluid power equation.
Shaft power formula
P_shaft = P_air / η
Divide air power by overall fan efficiency to get the motor/shaft power required to drive the fan.
Horsepower conversion
1 hp = 745.7 W
Used to express shaft power in horsepower for motor sizing.
Typical fan efficiency
40% – 75%
Axial and centrifugal fans typically fall in this range depending on design and operating point.

Your Results

Calculated
Air Power
-
P = Q × ΔP, useful power delivered to the airstream
Shaft (Brake) Power
-
Air power ÷ fan efficiency
Shaft Power (Horsepower)
-
Shaft power converted to hp (1 hp = 745.7 W)
Power Lost to Inefficiency
-
Shaft power − air power, dissipated as heat and turbulence

Ready

Enter airflow rate, pressure rise, and efficiency, then press Calculate.

Formula and Method for Fan Power Calculations

A fan does useful work by moving air against resistance — the static pressure of ducting, filters, grilles, or open space. The power a fan must deliver to the airstream, called air power (or fluid power), is the product of the volumetric flow rate and the pressure rise it produces: Pair = Q × ΔP, where Q is the flow rate in cubic meters per second and ΔP is the pressure rise in pascals, giving power directly in watts. Because no fan is perfectly efficient, the shaft power the motor must actually supply is higher: Pshaft = Pair / η, where η is the fan's overall aerodynamic and mechanical efficiency.

How the calculation works

Enter the fan's airflow rate and its unit (CFM, m³/s, m³/h, or L/s), the static pressure rise it produces and its unit (inches of water, pascals, millimeters of water, or psi), and the fan's overall efficiency as a percentage. The calculator converts both quantities to SI units — cubic meters per second and pascals — multiplies them to get air power in watts, then divides by the efficiency fraction to estimate the shaft (brake) power the motor must supply. Shaft power is also converted to horsepower (1 hp = 745.7 W) for a quick comparison against motor nameplate ratings, and the difference between shaft and air power shows how much power is lost to inefficiency.

Common mistakes

  • Confusing flow rate with velocity: CFM and m³/s measure the volume of air moved per unit time, not the air's speed through a duct — those are related but different quantities.
  • Mixing up pressure units: a "1" typed for pressure means something very different as inches of water (249 Pa) versus psi (6,895 Pa) — always double-check the unit dropdown matches your source data.
  • Assuming 100% efficiency: real fans lose a substantial share of shaft power to aerodynamic and mechanical losses; skipping the efficiency term badly understates the motor power actually needed.

Real-world applications

  • HVAC engineers use air power and shaft power to size supply and exhaust fan motors for a target airflow against known duct, filter, and damper static pressure.
  • Industrial ventilation and dust-collection systems use the same formula to confirm a fan-and-motor pairing can overcome system resistance at the required flow rate.
  • Energy audits compare a fan's calculated shaft power to its actual electrical draw (shaft power ÷ motor efficiency) to spot oversized motors or worn, inefficient fans.
  • Data center and server-room cooling designs use it to estimate the power budget needed to move enough air across a given pressure drop through racks and filters.

Frequently Asked Questions

What is fan air power, and how does it differ from shaft power?
Air power (or fluid power) is the useful power actually delivered to the moving air: Pair = Q × ΔP, the flow rate multiplied by the pressure rise. Shaft (brake) power is the mechanical power the motor must supply to the fan wheel, which is always higher than air power because of aerodynamic, mechanical, and drive losses: Pshaft = Pair / η. The ratio between them is the fan's overall efficiency.
What efficiency should I use for a typical fan?
Overall fan efficiency depends on design and operating point. Simple axial (propeller) fans often run 30-50% efficient, backward-curved centrifugal fans commonly reach 60-75%, and well-matched industrial fans can exceed 80% at their best efficiency point. A manufacturer fan curve gives the most accurate value for a specific unit and operating condition; 65% is a reasonable planning default when no curve is available.
How do I convert fan shaft power to motor size in horsepower or kilowatts?
Divide shaft power in watts by 745.7 to get horsepower, or divide by 1000 to get kilowatts. Because motors are sold in standard increments (1/4 hp, 1/2 hp, 1 hp, and so on), round the calculated shaft power up to the next standard motor size, and add margin if the fan may run at a higher-pressure, higher-flow point than assumed.
Why does static pressure rise matter more than airflow alone?
A fan moving a large volume of air against little resistance needs relatively little power, while the same airflow pushed through a restrictive duct, filter, or damper needs much more, because power scales directly with pressure rise (P = Q × ΔP). Ignoring system static pressure from ducts, grilles, filters, and elbows is a common reason an installed fan underperforms its rated airflow.

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