Estimate hydraulic power and required motor size in kW and HP from flow rate, pump head, and pump efficiency.
This centrifugal pump motor power calculator assumes water at standard density. Final motor selection should also consider starting method, duty cycle, altitude, liquid properties, and a real pump curve.
Pump motor sizing is often where hydraulic design meets electrical design. A pump may satisfy the required flow and head, but if the motor is undersized the system may fail to start, overload, or operate inefficiently. That is why a quick pump motor power calculation is useful during early design and equipment selection.
This centrifugal pump motor power calculator estimates hydraulic power from flow rate and head, then adjusts for pump efficiency and a motor service margin to give a more practical motor size in kilowatts and horsepower. It is useful for water pumps, booster pumps, irrigation systems, transfer pumps, and general utility pumping.
It is especially helpful for users who need to calculate motor size for a pump without manually converting flow units or deriving hydraulic power each time.
| Variable | Meaning | Unit |
|---|---|---|
| Q | Flow rate | m3/s |
| H | Total dynamic head | m |
| eta | Pump efficiency | % |
| Ph | Hydraulic power | kW |
| Pdesign | Recommended motor size after margin | kW or HP |
Flow rate can be entered in cubic meters per hour, liters per second, or US gallons per minute. Head is entered in meters, and efficiency is entered as a percentage. The result is shown in kilowatts and horsepower.
Flow = 35 m3/h, head = 28 m, pump efficiency = 72%, service margin = 115%
Converted flow = 0.00972 m3/s
Hydraulic power = 1000 x 9.81 x 0.00972 x 28 / 1000 = 2.67 kW
Motor power before margin = 2.67 / 0.72 = 3.71 kW
Recommended design motor size = 4.27 kW, or about 5.73 HP
Flow = 12 L/s, head = 18 m, efficiency = 68%, service margin = 115%
Converted flow = 0.012 m3/s
Hydraulic power = 2.12 kW
Recommended design motor size after efficiency and margin = about 3.59 kW, or 4.81 HP
This page assumes water-like fluid density and a simplified efficiency input. Real pump motor selection should also check the pump curve, NPSH, viscosity effects, starting torque, operating duty, and the preferred standard motor size available in the project region. Still, the calculator is very useful for a first-pass answer when a project team needs to estimate pump motor size quickly.