Pump Power & Efficiency Calculator
Find the shaft power (kW) needed to drive a pump given flow rate, head, fluid density, and combined pump-motor efficiency. Use it when selecting motors or verifying energy consumption for water supply, HVAC, or process systems.
About this calculator
The hydraulic power delivered to a fluid by a pump is: P_hydraulic = ρ × g × Q × H, where ρ is fluid density (kg/m³), g is gravitational acceleration (9.81 m/s²), Q is volumetric flow rate (m³/s), and H is total dynamic head (m). Because no pump or motor is perfectly efficient, the actual shaft power required is higher: P_shaft (kW) = (Q × H × ρ × 9.81) / (η_pump × η_motor × 1000), where Q is in m³/s. In this calculator, flow rate is entered in L/s, so it is first divided by 1,000 to convert to m³/s. Combined efficiency (pump × motor) can range from 0.50 for small units to 0.90 for large, well-specified centrifugal pumps.
How to use
A pump moves water (ρ = 1,000 kg/m³) at 25 L/s against a total head of 30 m. Pump efficiency = 0.75, motor efficiency = 0.92. Step 1 — Convert flow: 25 / 1,000 = 0.025 m³/s. Step 2 — Apply formula: P = (0.025 × 30 × 1,000 × 9.81) / (0.75 × 0.92) / 1,000. Step 3 — Numerator: 0.025 × 30 × 1,000 × 9.81 = 7,357.5 W. Step 4 — Denominator: 0.75 × 0.92 = 0.69. Step 5 — P = 7,357.5 / 0.69 / 1,000 ≈ 10.66 kW. Select a motor rated at least 11 kW.
Frequently asked questions
How do I calculate the power required for a centrifugal pump?
The required shaft power equals the hydraulic power divided by the product of pump and motor efficiencies: P = (ρ × g × Q × H) / (η_pump × η_motor). Hydraulic power represents the energy transferred to the fluid per second, while efficiency terms account for internal friction and electrical losses. Always add a service factor of 10–15% above the calculated value when selecting a motor to handle starting loads and process variations.
What is total dynamic head and why does it matter for pump selection?
Total dynamic head (TDH) is the equivalent height of fluid that the pump must lift, expressed in metres. It combines static elevation difference, pipe friction losses, minor losses from fittings and valves, and any pressure difference between source and destination. TDH is the single most important parameter in pump selection because power consumption scales directly with it. Underestimating TDH leads to insufficient flow; overestimating it wastes energy and can cause instability at the operating point.
Why does fluid density affect pump power consumption?
Power consumption is directly proportional to fluid density: pumping a dense fluid like brine (1,200 kg/m³) requires 20% more power than pumping fresh water (1,000 kg/m³) at identical flow and head conditions. This is because the pump must impart more kinetic and potential energy to heavier fluid per unit volume. When dealing with slurries, hot water (lower density), or chemical solutions, always use the actual fluid density rather than defaulting to the water value to avoid motor overload.