Pump Selector

Hydronic pump sizing — horsepower, motor, and operating point
Inputs
Results
Water Horsepower
—
WHP (hydraulic)
Brake Horsepower
—
BHP (shaft)
Motor Size
—
HP (next standard)
Input Power
—
kW
Full Load Amps
—
A (estimated)
Flow Rate
—
lb/hr
Specific Gravity
—
relative to water
Annual Energy
—
kWh/yr (8,760 hrs)
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About This Calculator

This pump selector sizes a hydronic pump from flow rate (GPM) and total dynamic head (TDH). It returns water horsepower, brake horsepower, the next standard NEMA motor size, input power in kW, estimated full load amps, mass flow in lb/hr, fluid specific gravity, and annual energy at continuous operation. It supports water and 30% ethylene or propylene glycol.

Fluid density is interpolated from temperature and corrected for glycol, so specific gravity — and therefore the required horsepower — reflects the actual fluid. Enter pump and motor efficiencies from the manufacturer’s curve to get a realistic operating point; the tool flags pump efficiency below 60%.

Formula & Method
Water HPWHP = GPM × TDH × SG ÷ 3,960
Brake HPBHP = WHP ÷ ηpump
Input powerkW = BHP × 0.7457 ÷ ηmotor
Full load ampsFLA = kW × 1000 ÷ (√3 · V · 0.85)

These are the standard pump-power relationships from ASHRAE hydronics and pump-manufacturer practice. The constant 3,960 = 33,000 ft·lb/min per HP ÷ 8.34 lb/gal of water; multiplying by specific gravity (SG = ρ ÷ 62.4) corrects for glycol. The factor 0.7457 converts HP to kW. Full load amps use the three-phase relation with an assumed 0.85 power factor — an estimate; confirm against the motor nameplate and NEC Article 430.

Frequently Asked Questions
What is the difference between water horsepower and brake horsepower?
Water horsepower (WHP) is the hydraulic power actually delivered to the fluid, computed from flow, head, and specific gravity. Brake horsepower (BHP) is the shaft power the motor must supply, equal to WHP divided by pump efficiency. BHP is always larger than WHP because no pump is 100 percent efficient; the difference is lost to friction, recirculation, and turbulence inside the pump.
Why is the constant 3,960 used in the horsepower formula?
The constant 3,960 converts gallons per minute and feet of head into horsepower for water. It comes from 33,000 ft-lb/min per horsepower divided by 8.34 lb per gallon of water (33,000 divided by 8.34 is about 3,960). Multiplying by specific gravity corrects the result for glycol or other fluids that are denser than water.
How is the motor size selected?
The tool divides brake horsepower by motor efficiency to get the required shaft input, then rounds up to the next standard NEMA motor size (such as 1, 1.5, 2, 3, 5, 7.5 HP and so on). Selecting the next standard size up provides operating margin so the motor is not loaded to 100 percent at the design point.
How are full load amps estimated?
Full load amps are estimated from input kW using the three-phase relation FLA = kW times 1000 divided by the square root of 3, times line voltage, times an assumed 0.85 power factor. This is an approximation for sizing; always confirm actual full load amps from the motor nameplate and apply NEC Article 430 for branch-circuit and overload protection.
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Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.