Pump Power Calculator.
Calculate hydraulic kW, required shaft power, and motor size from flow rate and total head. Works for centrifugal, positive displacement, and submersible pumps. Includes service factor and recommended motor rating.
Pshaft = Phydraulic / ηpump
Pmotor = Pshaft / ηmotor
where ρ = 1000 × SG kg/m³, g = 9.81 m/s², Q in m³/s, H in m
Pump Power
Pump sizing requires three sequential calculations: hydraulic power (work done on the fluid), shaft power (overcomes pump losses), and motor power (overcomes motor losses). Get any one wrong and you'll have an undersized — or oversized — installation.
Pump Power Formulas
Hydraulic kW = Q × H × ρ × g / 3,600,000
Shaft kW = Hydraulic kW / η_pump
Motor kW = Shaft kW / η_motor
Where Q = flow rate (m³/h), H = total head (m), ρ = density (kg/m³), g = 9.81 m/s², η = efficiency.
Worked Example
Water (ρ = 1000), Q = 50 m³/h, H = 30 m, η_pump = 65%, η_motor = 88%.
- Hydraulic = 50 × 30 × 1000 × 9.81 / 3,600,000 = 4.09 kW
- Shaft = 4.09 / 0.65 = 6.29 kW
- Motor input = 6.29 / 0.88 = 7.15 kW
- Select 7.5 kW motor
Total Head Components
Total head H is the sum of:
- Static lift — vertical distance from suction to discharge
- Pressure head — pressure required at discharge point
- Friction losses — pipe length, fittings, valves (often 20-40% of total)
- Velocity head — usually negligible
Pump Selection Cautions
- NPSH — Net Positive Suction Head must exceed pump's required NPSH or cavitation occurs.
- Operating point — pump should run near Best Efficiency Point (BEP), not on the extremes of its curve.
- Minimum flow — running below ~30% of BEP causes recirculation damage.
Related Tools
For motor sizing, see Motor HP Calculator. For pipe and fluid systems, hydraulic engineering references.