Water Pump Calculator

Water HP = (Flow rate in GPM x Total head in feet) / 3,960. Brake HP (actual motor size) = Water HP / Pump efficiency. For example, 50 GPM at 100 feet of head with 65% efficiency: Water HP = (50 x 100) / 3,960 = 1.26 HP. Brake HP = 1.26 / 0.65 = 1.94 HP. Select the next standard motor size above this value (2 HP).

Total dynamic head is the total equivalent height the pump must lift water, including static head (actual vertical lift), friction head (pipe friction losses), and pressure head (any required discharge pressure). TDH = Static head + Friction losses + Pressure head. Friction losses depend on pipe diameter, length, flow rate, and number of fittings.

Pump efficiency is the ratio of hydraulic output power to the input shaft power. Centrifugal pumps typically operate at 50-80% efficiency at their best efficiency point (BEP). Selecting a pump near its BEP minimizes energy consumption and wear. Submersible well pumps typically have combined pump and motor efficiencies of 55-70%.

Residential water supply systems typically require 5-15 GPM flow rate and 40-60 PSI pressure (92-138 feet of head). A 3/4 HP to 1 HP pump usually covers most homes with a pressure tank. Determine the peak demand (number of fixtures x fixture flow rates), static water level, well depth, and pipe losses to calculate TDH accurately.

GPM (gallons per minute) is used for pump sizing and most hydraulic calculations. GPH (gallons per hour) is used for slower processes like chemical dosing or irrigation scheduling. Convert: GPH = GPM x 60. Most pump curves are plotted in GPM versus total head (feet) and use pump efficiency curves to select operating point.