Pump Power Calculator
By Santaji GadepatilinUpdated October 27, 2026
The power a pump needs depends on how much liquid it moves, how high it lifts it and how efficient it is. This sets the motor size and the energy cost — and the type of pump matters too, as compared in centrifugal vs positive displacement pumps. This free pump power calculator solves for whichever value you leave blank and shows the formula and steps.
Shaft power (required) (kW) vs Flow rate (m³/h) — other inputs held at your current values.
How to use the pump power calculator
Enter the flow, total head, density and pump efficiency to get the hydraulic and shaft power. You can also solve for the flow or head a pump can deliver with a given motor.
Pump power formulas
Ph = ρ g Q H Pshaft = Ph ÷ η
Worked example
50 m³/h of water against 30 m head, pump efficiency 70%:
- Q = 0.01389 m³/s
- Ph = 998 × 9.81 × 0.01389 × 30 = 4.08 kW
- Pshaft = 4.08 ÷ 0.70 = 5.83 kW → a 7.5 kW motor is a typical choice
Typical pump efficiencies
| Pump type and size | Efficiency |
|---|---|
| Small centrifugal (< 5 kW) | 40 – 65% |
| Medium centrifugal | 65 – 80% |
| Large centrifugal | 80 – 90% |
| Positive displacement | 70 – 90% |
Motor selection
Select a motor larger than the shaft power, to cover end-of-curve operation and fluid density changes:
| Shaft power | Typical service factor | Motor size |
|---|---|---|
| Up to 7.5 kW | +20–25% | Next standard size up |
| 7.5–22 kW | +15% | Next standard size up |
| Above 22 kW | +10% | Next standard size up |
Standard IEC motor sizes include 0.75, 1.1, 1.5, 2.2, 3, 4, 5.5, 7.5, 11, 15, 18.5, 22, 30, 37, 45 and 55 kW.
Electrical input power
Pelectrical = Pshaft ÷ ηmotor
A 6 kW shaft load on a 90% efficient motor draws about 6.67 kW. Over 8,000 hours a year at 0.12 per kWh, that is about 6,400 in electricity, often more than the pump’s purchase price.
Affinity laws
For a centrifugal pump at different speeds:
| Quantity | Changes with speed |
|---|---|
| Flow | ∝ n |
| Head | ∝ n² |
| Power | ∝ n³ |
Reducing speed by 20% with a VFD cuts power to about 51% — a large energy saving compared with throttling a valve.
NPSH
To avoid cavitation, the net positive suction head available (NPSHa) must exceed the pump’s required NPSH (NPSHr), with a margin of at least 0.5–1 m.
Finding total head
Total head = static lift + pressure difference (as head) + friction losses. Use the Darcy–Weisbach calculator for the pipe friction part. To convert pressure to head, 1 bar ≈ 10.2 m of water.
Frequently asked questions
What is the difference between hydraulic and shaft power?
Hydraulic power is the energy delivered to the fluid. Shaft power is what the pump needs from the motor, which is higher because of pump losses (hydraulic power ÷ efficiency).
What motor size should I choose?
Pick the next standard motor size above the shaft power at the highest expected duty, often with a 10–15% margin.
What is total head?
The total energy the pump must add, expressed as a height of liquid. It includes static lift, pressure difference and friction losses.
How do I calculate pump power?
Hydraulic power = ρ × g × Q × H. Divide by pump efficiency for shaft power. 50 m³/h at 30 m of head with 70% efficiency needs about 5.8 kW.
What is a typical centrifugal pump efficiency?
About 50–75% for small pumps and 75–90% for large pumps at their best efficiency point.