Well Pump Horsepower Calculator
Calculate the required brake horsepower (BHP) and water horsepower (WHP) for a well pump based on flow rate, total dynamic head, and pump efficiency.
gallons per minute
feet
percent (1–100)
percent (1–100)
dimensionless (water = 1.0)
Formulas Used
Water Horsepower (WHP) — theoretical hydraulic power:
WHP = (GPM × TDH × SG) / 3960
Where 3960 is derived from: 33,000 ft·lbf/min ÷ 8.34 lb/gal (weight of one gallon of water).
Brake Horsepower (BHP) — power required at the pump shaft:
BHP = WHP / η_pump
Motor Input Horsepower (MHP) — power drawn from the power supply:
MHP = BHP / η_motor
Electrical Power (kW):
kW = MHP × 0.7457
Overall System Efficiency:
η_overall = η_pump × η_motor × 100
Assumptions & References
- Total Dynamic Head (TDH) includes static lift (depth to water + surface elevation), friction losses in pipes, fittings, and any pressure head required at the discharge point.
- The constant 3960 is the standard US unit conversion factor for GPM, feet, and horsepower (33,000 ÷ 8.34 ≈ 3960).
- Water specific gravity is 1.0 at ~60 °F (15.6 °C). Adjust for other fluids or temperatures.
- Typical centrifugal well pump efficiencies range from 50 % to 80 %; motor efficiencies typically range from 85 % to 95 %.
- Always select a motor with a nameplate rating at or above the calculated MHP to avoid overloading.
- Formula source: Groundwater and Wells, 3rd ed., Johnson Screens / Roscoe Moss Company; AWWA Manual M21.
- 1 HP = 0.7457 kW (IEEE/NIST standard).