Well Pump Sizing Calculator Formula

Understand the math behind the well pump sizing calculator. Each variable explained with a worked example.

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Formulas Used

Water Horsepower

water_hp = flow_gpm * total_head / 3960

Brake Horsepower

brake_hp = pump_efficiency > 0 ? flow_gpm * total_head / 3960 / (pump_efficiency / 100) : 0

Motor Power

motor_watts = pump_efficiency > 0 ? flow_gpm * total_head / 3960 / (pump_efficiency / 100) * 746 : 0

Variables

VariableDescriptionDefault
flow_gpmRequired Flow Rate(GPM)10
total_headTotal Dynamic Head(ft)200
pump_efficiencyPump Efficiency(%)60

How It Works

Well Pump Sizing

Water HP = GPM x TDH / 3960

Brake HP = Water HP / Efficiency

The constant 3960 comes from converting GPM and feet of head to horsepower units. Submersible pump efficiency is typically 50 to 70 percent. Multiply HP by 746 to get watts.

Worked Example

10 GPM pump with 200 ft TDH, 60% efficiency.

flow_gpm = 10total_head = 200pump_efficiency = 60
  1. 01Water HP = 10 x 200 / 3960 = 0.505 HP
  2. 02Brake HP = 0.505 / 0.60 = 0.84 HP
  3. 03Motor = 0.84 x 746 = 627 W
  4. 04Select a 1 HP submersible pump.

Frequently Asked Questions

What is total dynamic head for a well pump?

TDH equals well depth plus drawdown plus friction loss plus delivery pressure. For a 150 ft deep well with 20 ft drawdown, 10 ft friction, and 40 psi (92 ft) delivery, TDH = 272 ft.

How do I know the pump efficiency?

Check the manufacturer pump curve. Residential submersible pumps are typically 50 to 65 percent efficient at the design point.

Ready to run the numbers?

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