Pump Sizing & Power Calculator
Calculate hydraulic power, shaft power, and motor power requirements for centrifugal pumps. Enter your operating parameters below to get instant results with the engineering formulas explained.
Inputs
Water = 1000
Typical: 60–85%
Typical: 90–95%
Results
2.45 kW
Power transferred to the fluid
3.50 kW
Power at the pump shaft (input to pump)
3.81 kW
Electrical power drawn by motor
5 kW
15% safety margin applied
How It Works — The Engineering Formulas
1. Hydraulic Power
The theoretical power required to move the fluid at the given flow rate and head.
Where: ρ = density (kg/m³), g = 9.81 m/s², Q = flow rate (m³/s), H = head (m)
2. Shaft Power (Brake Power)
The actual power that must be delivered to the pump shaft, accounting for pump inefficiencies (internal losses, recirculation, disc friction).
Where: η_pump = pump efficiency (decimal, e.g. 0.70 for 70%)
3. Motor Input Power
The electrical power consumed by the motor, accounting for motor efficiency losses (copper losses, iron losses, mechanical losses).
Where: η_motor = motor efficiency (decimal, e.g. 0.92 for 92%)
4. Motor Selection — Safety Margin
When selecting a motor, a 15% safety margin is typically applied to account for variations in operating conditions, specific gravity changes, and starting torque requirements. The selected motor should be the next standard size above the calculated value.
Standard motor sizes (kW): 0.75, 1.5, 2.2, 4, 5.5, 7.5, 11, 15, 18.5, 22, 30, 37, 45, 55, 75, 90, 110, 132, 160, 200, 250
Important Considerations
- •This calculator provides a theoretical estimate only. Always consult the pump manufacturer's performance curve for accurate selection.
- •NPSH (Net Positive Suction Head) must be verified separately — insufficient NPSH causes cavitation, which destroys pump impellers.
- •For viscous fluids (oil, syrup), correction factors must be applied — the Hydraulic Institute Standards provide viscosity correction charts.
- •Variable speed drives (VSDs) can significantly reduce energy costs for variable-flow applications. Affinity laws state that power ∝ speed³.
- •Specific gravity affects power linearly — pumping a fluid with SG = 1.5 requires 1.5× the power of water at the same flow and head.