Pump Horsepower Calculator
Calculate water horsepower (WHP) and brake horsepower (BHP) for a pump given flow rate, head, specific gravity, and pump efficiency. Size motors correctly.
Results
What is it?
Water Horsepower (WHP) is the theoretical power required to move fluid at a given flow rate and head � it represents 100% efficient pumping. Brake Horsepower (BHP) is the actual shaft power needed, accounting for pump inefficiency. Motor input power adds motor inefficiency on top of BHP. Together, these values drive motor selection and energy budgeting.
How to use
Enter the design flow rate, total dynamic head (TDH), fluid specific gravity, and pump and motor efficiencies. TDH is the sum of static lift, friction losses in pipe and fittings, and velocity head. Read the pump curve at your operating point for efficiency. The calculator returns WHP, BHP, and total kW draw.
Example scenario
A chilled water pump moves 200 GPM against 80 ft TDH with water (SG=1.0). At 70% pump efficiency: WHP = 4.04, BHP = 5.77. With a 92% motor: motor input = 6.27 HP = 4.68 kW. Select a 7.5 HP motor for adequate service factor.
Pro tip
Always add a service factor when selecting a motor � a 1.15 SF motor can handle momentary overloads. If BHP exceeds 90% of motor nameplate HP, size up. Variable frequency drives (VFDs) can save 50�60% of pump energy in variable-flow systems because power varies as the cube of speed reduction.