Field calculator
Pump brake horsepower calculator (BHP)
Sizing a pump and its motor starts with the brake horsepower, the power the pump shaft actually demands. The formula is BHP = (gpm x total head x specific gravity) / (3960 x pump efficiency). Enter the flow in gallons per minute, the total dynamic head in feet (the static lift plus the friction losses through the pipe and fittings, not just the vertical rise), the pump efficiency as a percent (often 60 to 80), and the fluid specific gravity (1.0 for water, higher for denser fluids). The tool returns both the water horsepower, which is the useful hydraulic work, and the brake horsepower, which is larger because no pump is perfectly efficient. The motor is then sized above the brake horsepower, using the motor service factor or stepping up to the next standard size so it is not loaded to its limit. Read the brake horsepower against the manufacturer pump curve at the actual operating point, since efficiency changes along the curve, and confirm the head calculation, the curve, and the final motor selection with the manufacturer and the engineer.
Result
Pump brake horsepower: BHP = (gpm × total head in feet × specific gravity) / (3960 × pump efficiency). Enter the flow, the total dynamic head (the static lift plus the friction losses, not just the vertical rise), the pump efficiency (often 60 to 80 percent), and the fluid specific gravity (1.0 for water). The water horsepower is the useful hydraulic work; the brake horsepower is what the pump shaft actually demands once the pump efficiency is accounted for, and the motor is sized above the BHP using a service factor or the next standard motor size. Read the brake horsepower against the manufacturer pump curve at the operating point, since efficiency varies along the curve. This sizes the pump and motor only; confirm the head, the curve, and the motor selection with the manufacturer and the engineer.
Worked example
A pump moves 100 GPM of water against 80 ft of head at 70% efficiency. Size the motor.
- Flow100 GPM
- Total head80 ft
- Specific gravity1.0
- Efficiency70%
- Water HP = GPM × head × SG ÷ 3,960 = 100 × 80 × 1.0 ÷ 3,960 = 2.02 WHP.
- Brake HP = WHP ÷ efficiency = 2.02 ÷ 0.70 = 2.89 BHP.
About 2.89 BHP, so a 3 HP motor. Add margin for a non-overloading selection and the motor service factor.
anvilfield.com/calculators/pump-brake-horsepower-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.
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Pump BHP FAQ
How do you size a hydronic pump?
Size a hydronic pump on two numbers: the design flow in GPM from the load, and the system head in feet from the friction of the longest circuit at that flow. Pick the pump so that duty point lands near its best efficiency point, never oversized, with the manufacturer's curve controlling the selection.
What is NPSH?
NPSH is net positive suction head, the suction-side pressure margin that keeps water from boiling at the impeller. NPSH available, from the system, must exceed NPSH required, from the pump curve, with margin. Fall below it and the pump cavitates. Most NPSH problems are a clogged strainer, throttled valve, hot water, or lost loop pressure.
Why does my pump cavitate?
A pump cavitates when suction pressure drops below the water's vapor pressure, so vapor bubbles form and collapse on the impeller, sounding like pumping gravel. The cause is on the suction side: a clogged strainer, a throttled suction valve, an air-bound or low-pressure loop, or water near its vapor pressure. Clean and open the suction first.
Why align a pump and motor?
Shaft misalignment is a top cause of bearing and mechanical seal failure. Misaligned shafts load the bearings with a side force every revolution and deflect the seal faces open, so the pump quietly destroys itself while it runs. Correct soft foot first, then align offset and angle with a dial or laser tool, and re-check warm.
Circulator or base-mounted pump: which do I need?
A circulator hangs inline and handles smaller flows like residential loops and risers, often a sealed wet-rotor unit with no maintenance. A base-mounted pump sits on a foundation for the larger chilled, hot, and condenser water mains, and gets rebuilt rather than swapped. The design flow and whether the pump must be serviced in place decide it.