Pump Horsepower Calculator
What size pump and motor do you actually need? Enter flow and head (or pressure) and this gives hydraulic power, brake horsepower at your efficiency, the next standard motor size, and the yearly running cost.
Three Kinds of Pump Power
The horsepower question has three answers, and mixing them up is how pumps get undersized. Hydraulic power is the useful work done on the fluid. Brake power is what the pump shaft actually draws once you account for pump inefficiency — this is the number that sizes the motor. Motor input power is the electricity drawn, brake power divided by motor efficiency.
The Formula
Flow in US gallons per minute, head in feet. The constant 3960 rolls up the weight of water and the definition of horsepower. In SI it is simply hydraulic power = ρ g Q H, divided by efficiency.
Why the Next Size Up
You never run a motor at exactly 100 percent of its rating. Picking the next standard size above the brake power gives margin for a worn pump, a heavier-than-rated fluid, or a duty point that drifts. The calculator suggests that standard size automatically.
Frequently Asked Questions
What head should I enter?
Total dynamic head: the static lift plus every friction loss in the system at your flow. Size the pipe losses first, then bring the head here.
What pump efficiency is realistic?
Small centrifugal pumps run 50 to 70 percent; larger well-matched pumps reach 80 percent or more. Use the value from the pump curve at your duty point rather than a guess.
Why does specific gravity matter?
Power scales directly with fluid weight. Pumping a fluid denser than water (SG above 1) raises the horsepower in proportion.
Related calculators
- Pressure Drop Calculator — find the head loss this pump must overcome.
- Pipe Flow Calculator — flow, velocity and pipe sizing.
- Three-Phase Power Calculator — size the electrical side of the motor.
- Engineering Unit Converter — hp, kW, head and pressure.