Utility July 13, 2026 · 8 Min Read

Pump Calculator – Guide & Formulas

Calculate fresh water hydraulic horsepower (WHP) and electrical motor brake horsepower (BHP) from flow rates and dynamic heads.

Size industrial fluid pump motors. Input flow rates, total dynamic head, and pump efficiency ratings to compute required shaft mechanical brake horsepower (BHP) and operational kW.

Key Takeaway

Use the free Pump Calculator to calculate fresh water hydraulic horsepower (whp) and electrical motor brake horsepower (bhp) from flow rates and dynamic heads. Get instant results with step-by-step explanations.

How to Use the Pump Calculator

  1. Input the fluid volumetric flow rate in Gallons per Minute (GPM).
  2. Input the total dynamic head (vertical lift height plus friction losses) in Feet.
  3. Input estimated pump mechanical efficiency (standard is 60% to 80%).
  4. Review output water horsepower (WHP), required motor brake horsepower (BHP), and active kW.

The Formula

Water HP = (Flow * Head) / 3960 | Brake HP = Water HP / (Efficiency / 100)

Variable Definitions

  • Q: System flow rate measured in Gallons per Minute (GPM)
  • H: Total dynamic hydraulic head height in feet
  • SG: Specific gravity of fluid (1.0 for pure fresh water)
  • eta_pump: Mechanical efficiency factor of pump rotor assembly

Sizing a Municipal Water Pump

Determine required motor HP to pump 120 GPM over 60 feet of total dynamic head at 70% pump efficiency.

  1. Step 1: Compute ideal Water Horsepower (WHP): (120 × 60) / 3960 = 1.82 HP.
  2. Step 2: Factor in mechanical efficiency: 1.82 / 0.70 ≈ 2.60 BHP.
  3. Step 3: Convert required mechanical capacity to kilowatts: 2.60 × 0.7457 ≈ 1.94 kW.
  4. Step 4: Specifying a 3-HP commercial electric motor provides adequate safety margin.

Frequently Asked Questions

What is Total Dynamic Head (TDH)?

TDH represents the total equivalent height water must be lifted, combining physical elevation rise plus friction pressure losses from pipe walls, fittings, and bends.

How does fluid viscosity affect pump horsepower?

Thicker, more viscous fluids (like oils or sludge) increase friction losses, requiring higher motor brake horsepower to pump the same volume.