Pump Power Calculator
Calculate the shaft power a pump must deliver to move fluid against a given head at a specified flow rate and efficiency. Ideal for selecting pumps in water supply, HVAC, and process piping systems.
About this calculator
Pump power is the energy per unit time required to raise a fluid against gravity and overcome system resistance. The hydraulic power formula is: P = (ρ × g × Q × H) / η, where P is shaft power (W), ρ is fluid density (kg/m³), g is gravitational acceleration (9.81 m/s²), Q is volumetric flow rate (m³/s), H is total head (m), and η is pump efficiency expressed as a decimal. A pump-type multiplier can account for additional factors such as safety margins or multi-stage configurations. Efficiency accounts for mechanical and hydraulic losses inside the pump, so the actual power drawn from the motor will always exceed the theoretical hydraulic power. Accurate power sizing prevents motor overloading and reduces energy costs.
How to use
Suppose you need to pump water (ρ = 1,000 kg/m³) at Q = 0.05 m³/s against a total head of H = 20 m, with a pump efficiency of 75% and a pump-type factor of 1. P = (1,000 × 9.81 × 0.05 × 20 × 1) / (75 / 100) = 9,810 / 0.75 = 13,080 W ≈ 13.1 kW. Enter each value into the corresponding field and the calculator returns the required shaft power immediately, helping you select a motor of at least 13.1 kW.
Frequently asked questions
How does pump efficiency affect the power I need to supply?
Pump efficiency directly inflates the required input power: a pump that is only 60% efficient needs 67% more shaft power than a theoretically perfect pump for the same hydraulic output. Real centrifugal pumps typically operate between 60% and 85% efficiency at their best efficiency point (BEP). Running a pump far from its BEP causes efficiency to drop sharply, wasting energy and increasing operating costs. Always select a pump whose BEP flow rate closely matches your design flow rate to minimise power consumption.
What is total head in a pump system and how do I calculate it?
Total head is the equivalent height of fluid the pump must lift, expressed in metres, and it encompasses static head (physical elevation difference), friction losses in pipes and fittings, and any pressure difference between source and destination. It is calculated as H = static head + friction head + velocity head + pressure head. For most building water supply systems, friction losses and static head dominate. Accurate head calculation requires knowing pipe lengths, diameters, fittings, and fluid properties — underestimating head leads to an undersized pump that cannot meet flow targets.
Why does fluid density matter when calculating pump power?
Fluid density appears directly in the power formula, so denser fluids require proportionally more power to pump at the same flow rate and head. Water has a density of 1,000 kg/m³, but seawater is about 1,025 kg/m³, and some process chemicals or slurries can exceed 1,500 kg/m³. If you select a pump based on water calculations but pump a denser fluid, the motor may overload. Always use the actual fluid density at operating temperature, since density decreases as temperature rises for most liquids.