Pump horsepower calculator

What is pump horsepower?

Pump horsepower is a measure of the power involved in moving water through a system. It is not a single number. It comes in three forms, and each is larger than the one before it because energy is lost at every stage.

Water horsepower is the useful work delivered to the water itself. Brake horsepower is the power the pump shaft actually draws, and it is always higher than water horsepower because no pump converts 100 percent of shaft power into water movement. Motor horsepower is what the electric motor must supply, higher still because the motor itself is not perfectly efficient. Sizing a pump motor correctly means working through all three.

Why pump horsepower matters for water and wastewater utilities

For a water or wastewater utility, pumping is often the single largest electricity cost. An oversized motor wastes energy on every run and shortens equipment life. An undersized motor runs at its limit, overheats, and fails early. Getting the horsepower right is both an energy decision and a reliability decision.

Pump sizing is also a data problem. The flow and runtime figures that feed calculations like this one live in the same operational systems a utility uses for metering, billing, and work orders. A utility running its assets on connected water utility management software can pull real flow and runtime data instead of estimating by hand every cycle.

How this calculator works

The calculator uses the standard hydraulic power formulas. The constant 3960 converts gallons per minute and feet of head into horsepower.

How to use the calculator

Worked example

For a pump moving 500 gallons per minute at 120 feet of head, pumping water:

The rounding step matters. Standard motors come in fixed sizes, so you specify the next size up from your calculated figure so the motor is not run at its absolute limit.

What each input means

Not sure what efficiency to enter?

Each input changes the result, so it helps to know what each one represents before you trust the number:

Common mistakes when sizing a pump motor

Two errors show up again and again. The first is using static lift alone for head and ignoring friction loss, which understates the real power needed and leads to an undersized motor. The second is treating the calculated brake horsepower as the motor size, rather than dividing by motor efficiency and then rounding up to a standard size. Both mistakes push the motor to run harder than intended, which raises energy use and shortens its life.

Frequently Asked Questions

What is the difference between water, brake, and motor horsepower?

Water horsepower is the useful work delivered to the water. Brake horsepower is what the pump shaft draws, higher because of pump inefficiency. Motor horsepower is what the motor must supply, higher still because of motor inefficiency. This calculator returns all three.

What efficiency should I use if I do not know mine?

Use 70 percent for the pump and 90 percent for the motor as a starting point. For an exact result, read the pump efficiency from the pump curve at your operating point and the motor efficiency from the motor nameplate.

How do I pick the motor size from brake horsepower?

Divide brake horsepower by the motor efficiency to get motor horsepower, then round up to the next standard motor size. For example, 24 hp rounds up to a 25 hp motor so the motor is not run at its limit.

Related tools

Once you have sized the pump, these related calculators cover the rest of a water or wastewater system:

To see how a single platform manages the flow, runtime, and asset data behind calculations like these, explore SMART360 water utility management software.