How this calculator works
Hydraulic power is the rate at which the pump adds fluid potential energy. Divide it by pump efficiency to obtain shaft input power. Differential head includes the complete change in total head across the pump, not just static elevation. Motor electrical power requires another division by motor efficiency. The displayed specific speed uses RPM, m³/s and metres and is a unit-dependent metric index evaluated most usefully at the best-efficiency point.
Formula
Ph = rho g Q H; Pshaft = Ph/eta; nq = RPM sqrt(Q)/H^(3/4).
Worked example
At 0.01 m³/s, 10 m head, density 1000 kg/m³ and g=10 m/s², hydraulic power is 1000 W. At 50% pump efficiency, shaft power is 2000 W. A 4000 W shaft at the same efficiency and 20 m head supports 0.01 m³/s.
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Frequently asked questions
Which horsepower is displayed?
The unit selector uses mechanical horsepower, about 745.7 W per hp.
Does shaft power include motor losses?
No. Electrical input is greater if motor efficiency is below one.
Can I compare specific speeds from different unit systems?
Only after conversion. US customary and metric definitions give different numerical indices.