Pump Power Calculator

Calculate the shaft power a pump needs from flow rate, total head and pump efficiency.

Pump power (kW) = 9.81 × flow (m³/s) × head (m) ÷ efficiency. Lifting 50 L/s against 30 m of head at 70% efficiency needs about 21 kW of shaft power.

Calculator

Flow rate (L/s)Total head (m)Total dynamic head: lift plus friction and fitting losses.Pump efficiency (%)

Required pump power

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How it works

Hydraulic power is ρ·g·Q·H — for water that's 9.81 × flow in m³/s × head in metres (in kW). Dividing by the pump efficiency gives the shaft power the motor must deliver. Add motor and drive losses on top when sizing the electric motor.

Worked example

Flow 50 L/s = 0.05 m³/s. Hydraulic power = 9.81 × 0.05 × 30 = 14.7 kW. ÷ 0.70 efficiency ≈ 21.0 kW shaft power.

Assumptions & limitations

Uses water density (1,000 kg/m³). 'Total head' must include static lift plus all friction and fitting losses, not just the vertical rise. Size the motor above this figure for a safety margin and motor efficiency.

Frequently asked questions

What is total head?

Total dynamic head is the vertical lift plus the friction and fitting losses through pipes, valves and bends at your flow rate. Using only the vertical lift underestimates the power needed, often badly on long pipelines.

Is this the motor size I should buy?

It's the shaft power at the pump. Choose a motor above it to allow for motor efficiency, duty margin and the pump's efficiency at the actual operating point — commonly the next standard motor size up.

How do I get pump efficiency?

Read it from the manufacturer's pump curve at your duty point (flow and head). Many pumps run at 60–80% near their best efficiency point; well away from it, efficiency and this estimate both drop.

Last updated 21 June 2026

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