ToolnozaOverview
Pump power calculations separate useful hydraulic power from shaft power. Hydraulic or water horsepower is the energy rate delivered to the fluid. Brake horsepower is higher because a real pump is not 100% efficient.
Formula and calculation
For US units, hydraulic horsepower = GPM × head(ft) × specific gravity ÷ 3,960. Brake horsepower = hydraulic horsepower ÷ pump efficiency as a decimal. For water near ordinary conditions, specific gravity is approximately 1.0. The metric equivalent is based on P = ρgQH and then dividing by efficiency for shaft power.
Worked example
At 100 GPM, 100 ft of total head, SG = 1.0: hydraulic HP = 100 × 100 ÷ 3,960 = 2.53 HP. At 75% pump efficiency, brake HP = 2.53 ÷ 0.75 = 3.37 HP. Motor selection needs its own margin and motor-efficiency check.
How to use the calculator
Use actual operating flow and total dynamic head, not only static elevation. Enter specific gravity for fluids other than water and pump efficiency at the expected operating point. Pair this tool with the TDH calculator when head is not yet known.
Common mistakes and practical limits
Efficiency changes across the pump curve. Do not assume the best-efficiency-point value applies everywhere. Brake horsepower is not the same as electrical input power; motor and drive losses add more input. NPSH, cavitation, pipe losses and motor sizing remain separate checks.
Frequently asked questions
What is water horsepower?
It is the ideal hydraulic power transferred to water at the stated flow and head.
Why is brake horsepower higher?
Because brake horsepower accounts for pump inefficiency: BHP = hydraulic HP ÷ efficiency.
Technical reference
U.S. Department of Energy — Pump System Info Card
Continue with related tools
- Pump Head Tdh Calculator - Water Pump Npsh Calculator - Well Pump Size Calculator