What is a Pump Power Calculator?
The pump power calculator computes shaft power and motor sizing from flow rate and head, and checks specific speed.
How it's calculated
Water power Pw(kW) = ρ × g × Q × H ÷ 1000
Shaft power = water power ÷ pump efficiency
Motor size = shaft power × (1 + safety margin)
Variable definitions
| Variable | Meaning | Unit |
|---|---|---|
| ρ (rho) | Fluid density (water = 1,000) | kg/m³ |
| g | Gravitational acceleration | 9.8 m/s² |
| Q | Flow rate | m³/s |
| H | Total head | m |
Assumptions
The base formula assumes the fluid is water (density 1,000 kg/m³). For more viscous fluids like oil, pipe friction losses increase significantly, so the actual required shaft power can exceed the calculated value.
Worked example
Flow rate 0.05 m³/s, total head 20m, pump efficiency 70%
- Water power = 1,000×9.8×0.05×20÷1000 = 9.8 kW
- Shaft power = 9.8÷0.7 ≈ 14 kW
- With a 15% safety margin, motor size ≈ 16.1 kW → select the next standard size (e.g., 18.5 kW)
What is 'head'?
Head expresses how high a pump can push water, measured in meters — essentially pressure converted into an equivalent water column height. 10m of head ≈ about 1 bar.
Total head is the sum of three things: static head (the actual height to lift), friction loss (pressure lost through pipes, elbows, and valves), and discharge pressure (pressure needed at the destination). A common mistake is forgetting friction loss — in long, winding piping, friction loss can exceed the static head, and a pump sized only for height will fail to deliver.
Good to know
- Pumps push water, they do not suck it up. Suction relies on atmospheric pressure, so the theoretical maximum suction lift is about 10.3m — in practice, losses limit it to roughly 6–7m.
- Cavitation is the most common cause of pump failure. If suction-side pressure drops below the water's vapor pressure, bubbles form and then collapse violently against the impeller, eroding the metal — a rattling, gravel-like sound is a classic warning sign.
- NPSH (Net Positive Suction Head) quantifies the margin against cavitation — available NPSH must exceed required NPSH.
- By the affinity laws, reducing rotational speed reduces flow proportionally, head by the square, and power by the cube — which is why a variable-frequency drive cutting speed by just 20% can nearly halve energy consumption.
- Motors are typically sized with a 10–20% safety margin, since actual operating conditions can vary from design assumptions.
Frequently asked questions
Q. What pressure does 10m of head represent?
About 0.98 bar (≈1 kgf/cm²). A 10m water column corresponds to roughly 1 atmosphere.
Q. What pump efficiency should I assume?
It varies by type and size, but small centrifugal pumps are often 50–70% efficient, and larger ones can exceed 80%. Check the manufacturer's performance curve.
Q. How do I prevent cavitation?
Keep suction piping short and wide, minimize suction-side losses, and ensure adequate margin above the required NPSH.