Pump Power Calculator
Hydraulic power and motor size for pumps
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Need a fast answer on hydraulic power and motor size for pumps? Pump Power Calculator answers with the steps beside it. Fill in Flow Rate (L/s), Head (m) and Pump Efficiency (%) on Pump Power and read your answer immediately. In Pump Power the working sits beside the answer. Pump Power suits budgeting and planning checks. No sign-up is needed for Pump Power. The math runs on your device. Pump Power sits in the Engineering section next to free online pump power calculator and pump power calculator hydraulic power motor size. Pump Power has no learning curve. The fields are labeled. Pump Power is free, instant and local.
What the defaults need
20 litres per second raised through 30 m of head requires 5.89 kW of hydraulic power, and a 7.85 kW shaft input with a 75% efficient pump.
The gap between the two figures is the pump’s efficiency losses — the energy that goes into heating the water and the casing rather than lifting it.
The pump power formula
Hydraulic power comes from flow and head; shaft power divides by efficiency:
Hydraulic power = ρ × g × Q × H, and Shaft power = Hydraulic power ÷ Efficiency
20 L/s against 30 m of head at 75% efficiency: (1000 × 9.81 × 0.020 × 30) ÷ 0.75 = about 7.8 kW.
| Flow (L/s) | Head (m) | Hydraulic (kW) | Shaft @75% (kW) |
|---|---|---|---|
| 10 | 20 | 1.96 | 2.62 |
| 20 | 30 | 5.89 | 7.85 |
| 30 | 30 | 8.83 | 11.77 |
| 40 | 40 | 15.70 | 20.93 |
| 50 | 50 | 24.53 | 32.70 |
Lower efficiency means a bigger motor for the same duty point. Add a safety margin when sizing the motor. Related: voltage drop and engineering calculators.
The formulas
Hydraulic power = ρ g Q H, with ρ = 1,000 kg/m³ for water, g = 9.81 m/s², Q in m³/s and H in metres.
Shaft power = hydraulic power ÷ pump efficiency.
Worked example
Flow 20 L/s = 0.020 m³/s
Head 30 m
P = 1,000 × 9.81 × 0.020 × 30 = 5,886 W = 5.89 kW
Motor = 5.89 ÷ 0.75 = 7.85 kW
Choosing the motor
Pick the next standard motor size above the shaft power — 11 kW here rather than a 7.5 kW, because the calculated figure is already at the edge and starting current, viscosity and wear all consume margin. Standard sizes step 5.5, 7.5, 11, 15, 18.5, 22 kW.
What the calculation leaves out
- Friction losses. Pipe, fittings and valves add head that the static figure does not include — often 20–50% in a long system.
- Motor efficiency. A 75% pump with an 88% motor needs more electrical input than 7.85 kW; motor efficiency sits on top of pump efficiency.
- Duty point. A pump runs at its stated efficiency only near its best efficiency point; throttled or oversized it drops several points.
- Viscosity and temperature. Both change the power requirement, and a hot fluid can cavitate a pump sized for cold water.
Why the head figure matters more than flow
Power is linear in both, but head is the term people underestimate: a 30 m static lift with 15 m of friction is a 50% increase in motor size. Measure the actual system head, not just the vertical distance.
Frequently Asked Questions
Why 5.89 kW of hydraulic power?
ρgQH = 1,000 × 9.81 × 0.020 × 30 = 5,886 W. That is the power needed to lift 20 litres a second through 30 metres, before any pump losses.
Why is the motor 7.85 kW instead of 5.89?
Because a 75% efficient pump needs more shaft input than the hydraulic output it delivers: 5.89 ÷ 0.75 = 7.85 kW.
What motor size should I actually buy?
The next standard size above the shaft power — 11 kW here. The calculated figure leaves no margin for friction, wear or starting current.
Do I need to add pipe friction?
Yes. The head entered should be total system head, static lift plus friction losses. On a long run, friction can add 20–50% and change the motor size.
Does the efficiency figure include the motor?
No, it is the pump only. Motor efficiency of 85–90% applies on top, so the electrical input is higher than the shaft power calculated here.
How much power to move 10 L/s through 45 m?
1,000 × 9.81 × 0.010 × 45 = 4,414 W, so 4.41 kW hydraulic. At 75% efficiency that is 5.89 kW at the shaft — the same motor duty as the defaults, because half the flow at one and a half times the head balances out.
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