Pump Power Calculator
Enter flow, head and efficiency to get hydraulic power (water hp) and shaft power — the numbers for motor selection on any centrifugal pump.
Open calculatorLast updated: 2026-09-15
How the calculation works
- Hydraulic power is the useful energy given to the fluid: ρgQH — mass flow lifted through head H.
- Shaft power divides by pump efficiency; motor nameplate should exceed shaft power with a service factor margin.
Worked example
Interpreting the result
Head is total head: static lift plus every friction loss in pipes, fittings and valves. Sizing the motor for the duty point (not shutoff) prevents both overload at runout and waste at deadhead. For viscous fluids or slurries, efficiency drops well below the water-benchmark figure.
Assumptions
- Newtonian fluid, water-like behavior unless SG is changed.
- Steady duty point on the pump curve.
Limitations
- Viscosity corrections for oils and sludges are substantial — use pump manufacturer curves.
- NPSH and cavitation checks are separate concerns.
Shaft power (kW) for water at 70 % pump efficiency
| Flow | 10 m head | 20 m head | 40 m head |
|---|---|---|---|
| 50 L/min | 0.12 | 0.23 | 0.47 |
| 100 L/min | 0.23 | 0.47 | 0.93 |
| 200 L/min | 0.47 | 0.93 | 1.87 |
| 400 L/min | 0.93 | 1.87 | 3.74 |
Shaft power = ρ × g × Q × H ÷ efficiency, with water at a specific gravity of 1.
Worked scenario: 100 L/min against 20 m of head
The water itself receives 0.327 kW. A pump at 70 % efficiency needs 0.467 kW (0.63 hp) at its shaft. The motor is then chosen with a margin above that figure.
Mistakes to avoid
- Counting only the lift. Total head includes pipe friction, fittings, and any pressure the water has to reach.
- Ignoring the fluid. A denser liquid needs proportionally more power, which is what the specific gravity field is for.
- Using the pump's best-efficiency figure when it runs far from its design point.
Frequently asked questions
What is water horsepower?
The pure hydraulic output: flow × head × specific weight, with no losses. Shaft power = water hp ÷ pump efficiency; motor power adds motor efficiency too.
How much power to lift water 10 meters?
Per L/s: 0.098 kW hydraulic. At 10 L/s: 0.98 kW — before pump and motor losses, which roughly double the electrical draw in practice.