Pump Sizing Calculator — Head, Flow, Curve
Calculate the brake horsepower required from a pump given the flow rate, total head, and pump efficiency.
- Enter flow rate Enter Flow rate in GPM — the design flow the pump must deliver.
- Enter total head Enter Total head in feet, the total dynamic head the pump works against (use the Pump Head calculator if you need this value).
- Enter pump efficiency Enter Pump efficiency as a decimal from 0 to 1 (e.g. 0.6 for a typical small centrifugal pump).
- Calculate brake horsepower Press Calculate to get the brake horsepower, then round up to the next standard motor size (¼, ⅓, ½, ¾, 1, 1½ HP).
How do I calculate pump horsepower from GPM and head?
Brake horsepower is BHP = (GPM × head in feet) ÷ (3960 × efficiency). The 3960 constant is the imperial conversion factor for water (1 HP equals 550 ft·lbf/s at 62.4 lbf/ft³). This calculator computes hydraulic power first, then divides by pump efficiency to get the brake horsepower the motor must supply.
What does pump efficiency mean here?
Efficiency is the fraction of input shaft power that becomes useful hydraulic power, entered as a decimal between 0 and 1. Small centrifugal pumps often run around 0.5–0.7, so lower efficiency means more brake horsepower for the same flow and head. The figure should come from the pump curve at your design operating point.
Why round up to the next standard motor size?
Motors are manufactured in discrete sizes (¼, ⅓, ½, ¾, 1, 1½ HP and up), so you select the next size at or above the computed brake horsepower. You should also verify NPSH-available against the pump curve and aim to operate near the best-efficiency point to avoid overload and premature wear.