Fan Power Calculator (airflow & pressure → kW, brake horsepower, fan laws)

Understanding the fan power calculator — concept and principle

This calculator works out the power a fan needs from its airflow and pressure, the electricity it draws through its motor, its yearly energy use, and how much power you save by slowing it down. Facility managers, HVAC students and makers use it to size motors and estimate the payback of variable-speed drives.

From air power to the meter: with the default inputs the result is 0.77 kW input (1,543 kWh a year).

How it is calculated

Air power (W) = airflow (m³/s) × total pressure rise (Pa).
Shaft (brake) power = air power ÷ fan total efficiency.
Electrical input = shaft power ÷ motor and drive efficiency.
Brake horsepower = shaft power ÷ 745.7 W.
Energy per year (kWh) = input kW × hours per day × days per year.
Fan laws at a new speed: airflow × speed ratio; power × (speed ratio)³.

The principle

A fan does useful work by pushing a volume of air against a pressure. Multiplying airflow in cubic metres per second by pressure in pascals gives watts directly, because a pascal is a newton per square metre. Real fans lose energy in turbulence and friction, so the shaft must supply more than the air power, and the motor and belt lose a little more again. The fan laws describe one fan in one duct system: airflow is proportional to speed, pressure to speed squared and power to speed cubed. That cube is why turning a fan down a little saves a lot of energy.

Worked example

Default inputs: 3,000 m³/h, 500 Pa, fan efficiency 60%, motor 90%, 8 h/day, 250 days, new speed 80%.

Air power = 3,000 ÷ 3,600 × 500 = 417 W.
Shaft power = 416.7 ÷ 0.60 = 694 W = 0.69 kW = 0.93 hp.
Electrical input = 694.4 ÷ 0.90 = 772 W = 0.77 kW.
Energy = 0.7716 × 8 × 250 = 1,543 kWh a year; airflow = 3,000 ÷ 60 ÷ 0.0283 = 1,766 cfm.
At 80% speed: airflow 2,400 m³/h, power 0.7716 × 0.8³ = 0.395, shown as 0.4 kW.

Each point is the calculator re-run with only “Airflow (m³/h)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Airflow (m³/h)	Electrical input power (kW)
750	0.19
1,500	0.39
2,250	0.58
3,000	0.77
3,750	0.96
4,500	1.16
6,000	1.54

How to read the result

The electrical input is what you pay for. If the fan runs many hours, even a few percent of efficiency is worth a lot over a year. The speed-reduction line shows the potential of a variable-speed drive: if the system can work with 80% airflow, power falls to about half. In practice motor and drive efficiencies also drop at low load, so savings are a little smaller.

Common mistakes

Using static pressure when the efficiency is a total efficiency, or the reverse.
Mixing m³/h with m³/s.
Applying the fan laws after changing the ductwork or dampers; they assume the same system.
Sizing a motor exactly to the shaft power with no margin.

Keep in mind: Real fan selection needs the manufacturer's performance curves, and electrical work must follow local codes. Have a qualified professional design or change ventilation systems.

Frequently asked questions

How do I calculate fan power?

Multiply airflow in m³/s by pressure in Pa to get air power, then divide by the fan and motor efficiencies.

How much power does slowing a fan save?

Power falls with the cube of speed, so 80% speed needs about 51% of the power.

What is brake horsepower?

The power delivered to the fan shaft, in horsepower (1 hp = 745.7 W).

Sources: US DOE and AMCA International, "Improving Fan System Performance: A Sourcebook for Industry"

Checked 9 Nov 2026. US Department of Energy (EERE Industrial Technologies Program) and AMCA International, “Improving Fan System Performance: A Sourcebook for Industry”, Section 1 (total efficiency = total pressure × airflow ÷ (bhp × 6,362); fan laws: airflow ∝ speed, pressure ∝ speed², power ∝ speed³) and Section 2 (typical fan efficiencies).

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