Pump Horsepower Calculator

Pump efficiency (η) is the ratio of hydraulic power output to shaft power input. It accounts for all internal losses: disc friction (impeller face drag), recirculation losses, leakage past wear rings, and fluid friction in the volute. Efficiency is not a single number - it varies with flow rate and is plotted on the manufacturer's pump performance curve as a series of efficiency contour lines or a single curve at the design point. Peak efficiency typically runs 70–85% for end-suction centrifugal pumps and up to 90% for large axial-flow pumps. Always read efficiency at your actual operating flow rate, not at the BEP unless you are confident the operating point will remain at the design condition.

Total dynamic head (TDH) is the total equivalent height of liquid column the pump must produce to move fluid from suction to discharge against all resistances. It is the sum of static head (vertical lift), pressure head difference between suction and discharge tanks, friction head (pipe and fitting losses), and velocity head at the discharge. TDH is independent of the fluid's density - the same pump will develop the same TDH in feet whether pumping water or glycol - but the power required scales with SG because heavier liquids require more force to lift the same height.

Specific gravity appears directly in the power formula: HHP = (Q × H × SG) ÷ 3960. Pumping a fluid with SG = 1.3 (such as a 30% sodium chloride brine) requires 30% more horsepower than pumping pure water at the same flow and head. The pump impeller and casing are sized for the hydraulic performance (flow and head) and are largely indifferent to density, but the motor must supply the additional power for the heavier fluid. Pump data sheets typically state performance curves for water (SG = 1.0), so always multiply the tabulated BHP by the actual SG of your process fluid before selecting a motor.

A variable-frequency drive (VFD) is worth considering whenever pump flow must be modulated and the system head curve is dominated by friction (not static lift). Pump power follows the affinity laws: power scales as the cube of speed, so reducing speed to 80% cuts power consumption to 0.8³ = 51% of full-speed power. This makes VFDs extremely cost-effective for HVAC chilled-water pumps, irrigation systems, and process transfer pumps that run at partial load for many hours per day. VFDs are less beneficial when static head dominates, because slowing the pump quickly moves its operating point off the curve into shut-off, providing no useful flow reduction range.

Standard practice is to size the motor to at least 115–125% of the calculated BHP, then select the next standard NEMA frame motor size above that result. NEMA motors carry a service factor of 1.15 (meaning they can operate continuously at 15% overload without damage), but relying on the service factor as design margin shortens motor life and voids warranties. For continuous-duty pump applications, select a motor where the calculated BHP is no more than 90% of the motor's nameplate horsepower at the expected operating point. Add additional margin for motors that will experience frequent starting (induction surge current), high ambient temperatures, or operation at altitudes above 3,300 ft where air cooling is reduced.