Calculate water horsepower and brake horsepower for irrigation pumps from GPM, TDH, and pump efficiency. Size your motor correctly.
Sizing an irrigation pump motor begins with calculating two horsepower values: water horsepower (WHP) and brake horsepower (BHP). WHP is the theoretical minimum power needed to move water at a given flow rate and head. BHP accounts for pump losses and is always higher — it is the actual power the motor must deliver to the pump shaft.
The relationship is simple: BHP = WHP divided by pump efficiency. A pump rated at 75% efficiency requires 33% more horsepower than the theoretical minimum. Motor selection must meet or exceed BHP to avoid overloading.
This calculator computes both WHP and BHP and converts to kilowatts, giving you the numbers needed to select a motor from manufacturer catalogs. Whether you are a beginner or experienced professional, this free online tool provides instant, reliable results without manual computation. By automating the calculation, you save time and reduce the risk of costly errors in your planning and decision-making process.
An undersized motor overheats and fails prematurely. An oversized motor costs more and may operate at poor power factor. This calculator ensures you select the right motor size for your pump and operating conditions. Having a precise figure at your fingertips empowers better planning and more confident decisions. Manual calculations are error-prone and time-consuming; this tool delivers verified results in seconds so you can focus on strategy.
WHP = (GPM × TDH) / 3960 BHP = WHP / (Pump Efficiency / 100) kW = BHP × 0.746
Result: WHP = 50.5 HP; BHP = 67.3 HP (50.2 kW)
WHP = (800 × 250) / 3960 = 50.5 HP. BHP = 50.5 / 0.75 = 67.3 HP. In kW: 67.3 × 0.746 = 50.2 kW. A 75 HP motor provides adequate margin.
Always select the next standard motor size above BHP. For 67 BHP, use a 75 HP motor. This provides a safety margin for fluctuating conditions, start-up loads, and minor efficiency degradation over time.
Single-stage centrifugal pumps are used for low-head, high-flow applications. Multi-stage vertical turbine pumps add stages to develop high head for deep wells. Each stage adds about 25–40 ft of head. BHP increases linearly with stage count.
Three-phase motors at full load typically have 90–95% efficiency and 85–92% power factor. Running a motor at 50% load drops efficiency and power factor, increasing utility costs. Proper sizing ensures full-load operation.
WHP is the theoretical power needed to lift water at a given flow and head with zero losses. It represents 100% pump efficiency. Real pumps always require more power due to hydraulic, mechanical, and volumetric losses.
BHP is the actual power the motor must deliver to the pump shaft. It accounts for pump efficiency losses. BHP = WHP / pump efficiency.
WHP assumes a perfect pump. Real pumps lose 15–40% of input power to friction, leakage, and turbulence. A motor sized at WHP would be severely undersized.
The pump manufacturer's performance curve shows efficiency at various flow rates. A field pump test gives actual efficiency. New pumps: 70–82%; aged pumps: 50–65%.
It converts GPM and feet of head to horsepower: 1 HP = 33,000 ft·lbs/min. Since 1 gallon weighs 8.33 lbs, 33,000 / 8.33 = 3,960.
Size the motor frame and nameplate HP to match BHP. The motor's own efficiency determines the electric input (kW) from the grid, which you use for energy cost and breaker/wire sizing.