Pump Calculator

When to Use Each Variable

• Solve for Water HP — when you know the flow rate and total head and want the theoretical minimum power, e.g., establishing a baseline before accounting for efficiency losses.
• Solve for Brake HP — when you need the actual motor power required, e.g., selecting a pump motor that accounts for real-world efficiency losses.
• Solve for Efficiency — when you know both WHP and BHP and want to evaluate pump performance, e.g., monitoring efficiency degradation over time.
• Solve for NPSH — when checking whether the suction conditions will cause cavitation, e.g., verifying adequate suction head for a new pump installation.

Key Concepts

Water horsepower is the theoretical minimum power to move water at a given flow and head. Brake horsepower adds real-world losses (mechanical friction, hydraulic inefficiency). Pump efficiency is the ratio of useful hydraulic power to shaft input power. NPSH determines whether cavitation — vapor bubble formation that damages impellers — will occur at the suction side.

Applications

• Municipal water supply: sizing pump motors for water treatment plant distribution systems
• Irrigation engineering: selecting pumps to deliver required flow rates against elevation and friction losses
• Building services: choosing booster pumps for high-rise water supply with adequate NPSH margins
• Industrial processes: evaluating pump efficiency to reduce energy costs in continuous-operation plants

Common Mistakes

• Using the WHP value to select a motor — WHP is theoretical; you must use BHP (which is always higher) and then add a safety margin when choosing the motor size
• Ignoring NPSH requirements — if available NPSH drops below the pump's required NPSH, cavitation occurs and can destroy the impeller within months
• Assuming constant efficiency across the operating range — pump efficiency varies significantly with flow rate; operating far from the best efficiency point wastes energy and accelerates wear