Pump Selector

Pump horsepower, motor size, and annual energy from flow and total dynamic head.

Inputs
Results
Water Horsepower
—
WHP (hydraulic)
Brake Horsepower
—
BHP (shaft)
Motor Size
—
HP (next NEMA size ≥ BHP)
Input Power
—
kW
Motor FLA
—
A (NEC 430.250, 3-phase)
Flow Rate
—
lb/hr
Specific Gravity
—
relative to water
Annual Energy
—
kWh/yr at 8,760 h, design-point load
Inputs, results, and method — as an Excel workbook or plain CSV.

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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 smallest standard NEMA motor that covers the brake horsepower, input power in kW, the NEC Table 430.250 full-load current for that motor, mass flow in lb/hr, fluid specific gravity, and annual energy for the operating hours you enter. It supports water and 30, 40, and 50% (by volume) 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
Motor sizesmallest NEMA HP ≥ BHP
Full load ampsNEC Table 430.250 (motor HP, voltage, 3-phase)
Running currentI ≈ kW × 1000 ÷ (√3 · V · 0.85)
Annual energykWh = kW × operating hours

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 ÷ 8.33 lb/gal of water; multiplying by specific gravity (SG = ρ ÷ 62.4) corrects for glycol, with TDH in feet of the pumped fluid. Fluid density uses the same property data as the Hydronic Pipe Sizer (IAPWS water; MEGlobal and Melinder glycol). The factor 0.7457 converts HP to kW. Motor nameplate horsepower is shaft output, so the motor is the smallest standard size at or above BHP; motor efficiency sets the electrical input, not the nameplate. Full-load amps are the NEC Table 430.250 values that NEC 430.6(A)(1) requires for conductor and overcurrent sizing; the running current at the duty point is an estimate with an assumed 0.85 power factor. Annual energy assumes the pump runs at the design point for every operating hour, so it is an upper bound for variable-speed systems.

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 rounds brake horsepower up to the next standard NEMA motor size (such as 1, 1.5, 2, 3, 5, 7.5 HP and so on). Motor nameplate horsepower is shaft output, so it is compared with BHP directly; motor efficiency affects the electrical input in kW and amps, not the nameplate size. Designers usually also confirm the motor is non-overloading at the end of the pump curve, which can move it up one size.
Where do the full load amps come from?
Full load amps come from NEC Table 430.250 (three-phase induction motors) for the selected motor size and voltage, which is the value NEC 430.6(A)(1) requires for sizing conductors and branch-circuit protection. The tool also estimates the running current at the duty point from input kW with an assumed 0.85 power factor. Use the motor nameplate current for overload relay sizing.
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AIM Works · launches Oct 15, 2026

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How the calculations work →

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.