Pump Power Calculator
The Pump Power Calculator works out the hydraulic (water) power a pump adds to a fluid, the shaft (brake) power its motor must supply, and — if you enter a motor efficiency — the electrical input power drawn. Enter the flow rate, head, fluid density and pump efficiency to size a pump duty or estimate running power.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the flow rate Q in litres per second and the total head H in metres (use the total dynamic head from a pump-head calculation, not just the static lift).
- Adjust the fluid density if you are not pumping cold water (1000 kg/m³), and set the pump efficiency from the manufacturer's curve at your duty point.
- Optionally add the motor efficiency to get the electrical input power, then read the hydraulic, shaft and motor-input power results.
How it works
Hydraulic power is Ph = ρ·g·Q·H, with Q converted from L/s to m³/s (÷1000) and the result divided by 1000 to give kilowatts. This is the useful power transferred to the fluid. The shaft (brake) power is Ph divided by the pump efficiency, accounting for hydraulic and mechanical losses inside the pump. Dividing the shaft power by the motor efficiency gives the electrical power the motor draws from the supply. The calculator uses g = 9.81 m/s².
Worked example
Worked example. For Q = 20 L/s, H = 30 m, water (1000 kg/m³) and a 70% pump: Q = 0.02 m³/s, so Ph = 1000 × 9.81 × 0.02 × 30 ÷ 1000 = 5.886 kW. Shaft power = 5.886 ÷ 0.70 = 8.409 kW. Adding a 90% motor gives 8.409 ÷ 0.90 = 9.343 kW electrical input.
Common mistakes
- Using only the static lift for head — you must use the total dynamic head, which also includes friction and fitting losses.
- Confusing hydraulic power with shaft power; the motor must deliver the larger shaft power, not the hydraulic figure.
- Entering efficiency as a decimal (0.7) instead of a percentage (70), or reading efficiency at the wrong point on the pump curve.
Frequently asked questions
What is the difference between hydraulic, shaft and motor input power?
Hydraulic power is the useful power added to the water; shaft (brake) power is what the motor delivers to the pump after internal pump losses; motor input power is the electrical power drawn from the supply after motor losses. Each step is larger than the last.
What pump efficiency should I use?
Read it from the manufacturer's pump curve at your actual flow and head. Small pumps may be 40–60% efficient while large well-selected pumps can exceed 80%. The 70% default is a rough mid-range figure only.
Related tools
- Pump Head Calculator
- Pipe Flow Calculator
- Friction Loss Calculator
- Pipe Velocity Calculator
- Pump Affinity Laws Calculator
- Darcy-Weisbach Head Loss Calculator
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Tip: Enter any known values to calculate the remaining results.
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