Pump Horsepower Calculator

About the Pump Horsepower Calculator

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 page turns flow, head, and efficiency into the motor size range needed for the pump instead of leaving it as a catalog guess.

When This Page Helps

Motor sizing errors are expensive either way. This page helps compare required horsepower with the motor you plan to install.

How to Use the Inputs

1. Enter the pump flow rate in GPM.
2. Enter the total dynamic head (TDH) in feet.
3. Enter the pump efficiency as a percentage.
4. Read the water horsepower (WHP) and brake horsepower (BHP).
5. Select a motor rated at or slightly above BHP.

Example Calculation

Tips & Best Practices

• Always select a motor with 10―20% margin above calculated BHP.
• Standard motor sizes: 25, 30, 40, 50, 60, 75, 100, 125, 150, 200 HP.
• Higher pump efficiency directly reduces BHP and motor cost.
• Multi-stage turbine pumps have different efficiency at each stage count.
• Recalculate BHP if GPM or TDH changes due to well conditions.
• Motor efficiency (typically 90–95%) further affects energy input from the grid.

Motor Selection Guidelines

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.

Multi-Stage vs Single-Stage Pumps

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.

Power Factor and Motor Efficiency

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.

Frequently Asked Questions

• What is water horsepower?

  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.

• What is brake horsepower?

  BHP is the actual power the motor must deliver to the pump shaft. It accounts for pump efficiency losses. BHP = WHP / pump efficiency.

• Why not just use WHP to size the motor?

  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.

• How do I find pump efficiency?

  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%.

• What is the 3960 constant?

  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.

• Should I use BHP or electric input to size the motor?

  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.