Pump Power Calculator (Shaft Power Formula)
Verified by Rachel Mayfield, Supply Chain Analyst - April 2026
Calculate the shaft power (kW and hp) required to pump a given flow against a given head for any fluid. Pump Power Calculator (Shaft Power Formula) Uses the standard hydraulic power formula with an efficiency factor to account for pump internal losses. Typical pump efficiencies range from 40% for small circulators to 80-85% for well-matched end-suction pumps at design duty.
Calculate shaft power required
Formula: P_shaft (W) = rho x g x Q x H / eta. Uses g = 9.81 m/s^2. Hydraulic (water) power is the useful output before efficiency losses. For electric motor sizing add a safety margin (typically 10-20%) and check the motor service factor.
How to calculate pump shaft power
Formula: P = rho x g x Q x H / eta, where rho is fluid density (1000 kg/m^3 for water), g is gravitational acceleration (9.81 m/s^2), Q is volumetric flow (m^3/s), H is total head (m), and eta is pump efficiency (dimensionless). The result is in watts; divide by 1000 for kW.
Total head includes static lift, pipe friction losses, and any pressure required at the discharge. For closed-loop systems like heating circuits only the friction component matters since the static head cancels. For open systems like borehole pumps, static head is the vertical distance from the water surface to the discharge point.
Typical efficiencies: circulator pumps 40-60%, end-suction centrifugal 65-80%, split-case centrifugal 75-85%, submersible borehole 60-70%, positive displacement 70-90%. Always use the efficiency at the actual duty point, not the best efficiency point, since operating away from BEP reduces efficiency significantly.
References
- ISO 9906:2012 - Rotodynamic pump performance testing
- Europump Guide - Pump efficiency reference
Frequently Asked Questions
How do I calculate pump power in kW?
Shaft power (W) = rho x g x Q x H / eta, where rho is density (kg/m^3), g is 9.81 m/s^2, Q is flow in m^3/s, H is head in metres, and eta is pump efficiency. Divide by 1000 for kW. For water at 50 m^3/h and 30 m head at 70% efficiency: P = 1000 x 9.81 x 0.0139 x 30 / 0.70 = 5.83 kW.
What is a good pump efficiency?
Typical pump efficiencies: small circulators 40-60%, end-suction centrifugal pumps 65-80%, split-case centrifugal 75-85%, submersible borehole 60-70%, positive displacement 70-90%. Use the efficiency at actual duty, not the peak catalogue value.
What size motor do I need for my pump?
Calculate the shaft power required, then add a safety margin. For general industrial use, add 15-25% and round up to the nearest standard motor size. Check the motor service factor (typically 1.15) and the pump curve at the maximum possible duty point, not just design duty, since the motor must handle off-design operation.
What does the Pump Power Calculator (Shaft Power Formula) help engineers calculate?
The Pump Power Calculator (Shaft Power Formula) helps UK engineers calculate the shaft power required to pump a given flow against a given head for any fluid, using the standard hydraulic power formula with an efficiency factor.
Can you provide examples of typical efficiencies used in the Pump Power Calculator (Shaft Power Formula)?
The Pump Power Calculator (Shaft Power Formula) typically uses pump efficiencies ranging from 40% for small circulators to 80-85% for well-matched end-suction pumps at design duty.
How do I choose between different variants of the Pump Power Calculator (Shaft Power Formula)?
Choose a variant based on specific application needs, such as pump type and fluid properties. For instance, if you're dealing with high viscosity fluids or large flow rates, consider using versions tailored to those requirements.
What UK standards should I be aware of when using the Pump Power Calculator (Shaft Power Formula)?
When using the Pump Power Calculator (Shaft Power Formula) in the UK, adhere to BS EN 735-1 for pump performance and testing standards, ensuring accurate calculations and compliance with industry regulations.
Are there specific materials recommended for components used in pumps calculated by the Pump Power Calculator (Shaft Power Formula)?
Components like impellers and casings are often made from corrosion-resistant materials such as stainless steel or bronze to ensure longevity and efficiency when using the Pump Power Calculator (Shaft Power Formula).
Where can I find UK suppliers who offer pumps designed with calculations from the Pump Power Calculator (Shaft Power Formula)?
Look for UK-based pump manufacturers like Grundfos, KSB Pumps, and Blackmer, which provide pumps tailored to specifications calculated by the Pump Power Calculator (Shaft Power Formula), ensuring optimal performance and efficiency.
What is the approximate cost range and availability of pumps designed using the Pump Power Calculator (Shaft Power Formula)?
The cost for pumps designed with calculations from the Pump Power Calculator (Shaft Power Formula) varies widely, starting at around £500 for small circulators to over £10,000 for large industrial models. Availability depends on specific model and can be checked through UK suppliers like Armstrong International or EDDY Pump Corporation.