Calcmeister

Pump power calculator

Enter flow, head and the efficiencies. The calculator works out the power the pump needs and suggests a standard motor size.

m³/h

1 l/s = 3.6 m³/h. 1 US gpm = 0.227 m³/h.

m

Static lift plus friction losses in the pipework. 10 m of water is about 1 bar.

kg/m³
%

Read it from the pump curve at the duty point. Small pumps 40 to 65 %, large ones 75 to 90 %.

%
Shaft power
– kW
Hydraulic power
– kW
Electrical input
– kW
Suggested motor
– kW

    How pump power is calculated

    A pump lifts a mass of fluid against a head every second. The power it puts into the fluid is:

    Phyd = ρ · g · Q · H

    Pshaft = Phyd ÷ ηpump

    Pel = Pshaft ÷ ηmotor

    ρ
    fluid density in kg/m³
    g
    9.81 m/s²
    Q
    flow rate in m³/s (m³/h ÷ 3,600)
    H
    total head in metres
    η
    efficiency of pump and motor

    Worked example

    A pump moves 50 m³/h of water against 30 m of head. The hydraulic power is 998 × 9.81 × 0.0139 × 30 = 4.08 kW. At 70 % pump efficiency the shaft needs 5.83 kW. With a 15 % margin, the next standard motor is 7.5 kW.

    Finding the total head

    Total head is the sum of three parts:

    1. Static head: the height difference between the liquid surface on the suction side and the discharge point.
    2. Pressure head: any pressure difference between the two tanks, converted to metres (1 bar ≈ 10.2 m of water).
    3. Friction head: losses in pipes, bends, valves and filters. These grow roughly with the square of the flow.

    The pump runs where its curve meets the system curve. Use the head at that point, not the maximum head printed on the nameplate.

    How the motor size is suggested

    The calculator adds 15 % to the shaft power up to 22 kW and 10 % above that, then picks the next size from the IEC standard series: 0.37, 0.55, 0.75, 1.1, 1.5, 2.2, 3, 4, 5.5, 7.5, 11, 15, 18.5, 22, 30, 37, 45, 55, 75, 90 kW and up. Check the pump curve: if the pump can run further out on its curve, the power at the end of the curve should decide the motor.

    Unit conversions

    Questions and answers

    What is the formula for pump power?

    Hydraulic power P = ρ · g · Q · H, with density ρ in kg/m³, g = 9.81 m/s², flow Q in m³/s and head H in m. The result is in watts. Divide by the pump efficiency to get the shaft power.

    What is the difference between hydraulic power and shaft power?

    Hydraulic power is the energy the pump actually puts into the fluid. Shaft power is what the motor must deliver to the pump shaft. The difference is lost as heat and friction inside the pump.

    Why add a margin to the motor size?

    The pump may run to the right of its duty point, the fluid may be heavier, or wear may lower efficiency. A margin of 10 to 20 % keeps the motor from overloading. Check the end of the pump curve for non-overloading designs.

    How do I convert pressure to head?

    Head in metres = pressure in bar × 10.2 ÷ specific gravity. For water, 1 bar is about 10.2 m of head.

    Related calculators

    Last reviewed: 2026-10-14. Results are estimates for planning. Check them before you rely on them for design, purchase or safety decisions.