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.
Last 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.
Formula
P_hydraulic = ρ × g × Q × H P_shaft = P_hydraulic / η
| Symbol | Meaning | Unit |
|---|---|---|
P_h | Hydraulic (water) power | W |
P_s | Shaft (brake) power | W |
Q | Volumetric flow | m³/s |
H | Total head | m |
Worked example
100 L/min at 20 m head, 70% efficient pump: hydraulic = (0.001667 m³/s × 9810 × 20) = 327 W; shaft = 467 W → a 0.75 kW motor with service factor, or a 1/2 hp motor is marginal.
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.
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.