Pump Horsepower Calculator — Water HP, BHP & Motor Power
Find the hydraulic power a pump must deliver (water HP), the shaft power it draws (brake HP) and the motor size you need — in both horsepower and kilowatts — for any liquid, head and efficiency.
Unit system
GPM
ft
%
%
Shaft power the motor must supply to drive the pump at rated conditions
- 1
Water horsepower
(200 × 100 × 1) ÷ 3960 = 5.051 HP3960 is the unit-conversion constant for GPM, feet and HP. - 2
Brake horsepower
5.051 ÷ (75 ÷ 100) = 6.734 HP - 3
Motor horsepower
6.734 ÷ (90 ÷ 100) = 7.48
How does this calculator work?
Pump motor power = (Flow × Head × SG) / 3960 ÷ pump_efficiency ÷ motor_efficiency (US: GPM, feet). Water HP is the useful hydraulic power; brake HP is the shaft power drawn; motor HP is the electrical input. Enter flow, head, fluid SG and both efficiencies to get all three power figures in HP and kW.
Formula
How this is calculated
Pump power calculations start with the hydraulic (water) horsepower — the power actually delivered to the fluid. In US customary units it is WHP = (Q × H × SG) / 3960, where Q is the flow rate in US gallons per minute, H is the total dynamic head in feet and SG is the specific gravity of the fluid (1.0 for water at 15 °C). In SI the same quantity is Hydraulic Power (kW) = Q(m³/s) × H(m) × SG × 9.81. The divisor 3960 is a unit-conversion constant derived from 33 000 ft·lbf/min per HP and 8.34 lbm/gal for water.
The pump's hydraulic efficiency converts shaft input power (brake horsepower) to useful hydraulic output. Centrifugal pumps are typically 60–85% efficient; higher-efficiency impeller designs and operating near the best-efficiency point (BEP) approach the upper end. BHP = WHP ÷ η_pump. Finally, the driving motor also has losses, and Motor HP = BHP ÷ η_motor.
Efficiencies in this calculator are user-editable estimates — check the manufacturer's pump curve for the actual efficiency at your operating point. Always size the motor with a service factor (typically 1.15–1.25) above the calculated motor HP to accommodate starting torque, wear and off-design operation.
Frequently asked questions
Water HP (WHP) is the useful power delivered to the fluid — it depends only on flow, head and fluid density. Brake HP (BHP) is the shaft power the pump absorbs from the motor; it is always larger than WHP because pump inefficiencies (friction, turbulence, recirculation) consume extra energy.
Total dynamic head (TDH) is the sum of the static lift (elevation difference), velocity head at the discharge, and all friction losses in the piping system. You size the pump to overcome TDH at the required flow — use a hydraulic friction calculator or pipe-loss chart to estimate it.
Yes. Replace the constant 3960 with 3960 / SG in the denominator, or equivalently multiply the water-based WHP by SG. This calculator applies SG automatically — enter 1.2 for brine or 0.72 for diesel to get the correct power for that fluid.
Also known as
TG we-Calculate Editorial Team. (2026). Pump Horsepower Calculator — Water HP, BHP & Motor Power [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/pump-horsepower-calculator
TG we-Calculate Editorial Team. "Pump Horsepower Calculator — Water HP, BHP & Motor Power." TG we-Calculate. 2026. https://we-calculate.com/calculator/pump-horsepower-calculator.
TG we-Calculate Editorial Team, "Pump Horsepower Calculator — Water HP, BHP & Motor Power," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/pump-horsepower-calculator
@misc{wecalculate_pump_horsepower_calculator, title = {Pump Horsepower Calculator — Water HP, BHP & Motor Power}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/pump-horsepower-calculator}}, year = {2026}, note = {TG we-Calculate} }
Did this calculator help you?