Pump Power Calculator
Calculate the shaft power a centrifugal pump motor must deliver to move fluid against a given head at a specified flow rate. Use it when selecting pump motors or auditing energy consumption in piping systems.
About this calculator
The hydraulic power needed to lift or pressurise a fluid is given by P_hydraulic = ρ × g × Q × H, where ρ is fluid density (kg/m³), g = 9.81 m/s², Q is volumetric flow rate (m³/s), and H is total head (m). Because no pump is perfectly efficient, the shaft power drawn from the motor is higher: P_shaft = P_hydraulic / η, where η is the pump efficiency (expressed as a decimal). The full formula implemented here is P_shaft (kW) = (Q/3600) × H × ρ × 9.81 / (η × 1000), converting flow from m³/h to m³/s and watts to kilowatts. Total head H includes static lift, friction losses, and velocity head. Efficiency η for centrifugal pumps typically ranges from 0.50 to 0.85 depending on design and operating point.
How to use
Size a pump motor for: Flow rate = 50 m³/h, Total head = 30 m, Water density = 1,000 kg/m³, Pump efficiency = 70% (0.70). Step 1 — convert flow: 50 / 3600 = 0.01389 m³/s. Step 2 — hydraulic power: 0.01389 × 30 × 1,000 × 9.81 = 4,087 W. Step 3 — shaft power: 4,087 / 0.70 = 5,838 W = 5.84 kW. Select a standard motor of at least 5.84 kW (typically the next standard frame, e.g. 7.5 kW, to allow for motor efficiency and start-up loads).
Frequently asked questions
How does pump efficiency affect the motor power I need to buy?
Pump efficiency directly scales the required motor power — a pump at 60% efficiency needs 67% more shaft power than a theoretically perfect pump for the same hydraulic output. For example, moving from a 60% to a 75% efficient pump at the same duty point reduces motor power demand by 20%, cutting energy costs significantly over a pump's lifetime. This is why operating centrifugal pumps near their Best Efficiency Point (BEP) is so important. Variable-speed drives can maintain near-BEP operation across varying flow demands, further reducing energy use.
What is total head in a pump system and how do I calculate it?
Total head is the equivalent height of fluid column that the pump must overcome, expressed in metres. It is the sum of static head (actual elevation difference between suction and discharge), friction head losses in pipes and fittings (calculated using the Darcy-Weisbach or Hazen-Williams equations), and velocity head differences. Pressure differences at suction and discharge boundaries also contribute. Accurately estimating all these components is essential — underestimating head leads to insufficient flow, while overestimating it results in an oversized, inefficient pump selection.
Why does fluid density matter when calculating pump power requirements?
Pump head is defined in metres of the pumped fluid, so a denser fluid requires proportionally more force — and therefore more power — to achieve the same head. Water at 1,000 kg/m³ is the standard reference, but pumping a fluid such as fuel oil at 850 kg/m³ requires 15% less power, while pumping a brine at 1,200 kg/m³ requires 20% more power for the same head and flow. Pump curves published by manufacturers are typically based on water; always correct for density when specifying motors for non-water fluids.