Pump power calculator
Calculate the hydraulic power delivered to water and the shaft power a motor needs to drive the pump.
How it is calculated
Hydraulic power Ph = ρ × g × Q × H, with ρ = 1000 kg/m³ and g = 9.81 m/s² (gives kW when Q is in m³/s). Shaft power Ps = Ph ÷ η.
Worked example
Q = 20 L/s, H = 30 m, η = 75 %: Ph = 9.81 × 0.020 × 30 = 5.89 kW, Ps = 5.89 ÷ 0.75 = 7.85 kW.
Steps
- Enter the design flow rate in L/s.
- Enter the total head (static lift plus friction losses).
- Enter the pump efficiency from its curve, then pick a motor one size above the shaft power.
Frequently asked questions
What is total head?
Static head (height difference) plus friction losses in the piping, at the design flow.
Which motor should I choose?
Pick a motor rated above the shaft power with a margin, typically 10 % to 15 %, and check the service factor.
Does this work for other liquids?
Only for water. For other liquids, multiply by the specific gravity.
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References
Pump hydraulic and shaft power from ISO 9906 (rotodynamic pump acceptance tests) and standard pump design (Hydraulic Institute).
Estimates for information only. Verify critical values with a qualified engineer or your supplier standards.