Well Pump Sizing Calculator by GPM & Horsepower

Sizing a well pump means matching two things: how much water the house demands at peak, and how hard the pump must work to deliver it. Peak demand is estimated from bathroom count — a common rule of thumb starts around 6 GPM for a one-bathroom home and adds roughly 2 GPM per additional bathroom, standing in for the fixtures likely to run at once.

The work side is total dynamic head — the vertical distance the pump lifts water plus the pressure it must maintain at the house. Depth is added directly in feet, and system pressure converts at 2.31 feet of head per psi. With flow and head known, water horsepower follows from the hydraulic identity GPM times head divided by 3,960, and dividing by the pump-and-motor efficiency gives the brake horsepower the motor must actually produce.

Treat every number here as a planning estimate. Real well yield, pipe friction, drawdown, and pressure-tank settings all shift the answer, and an undersized or oversized pump both cause problems. Use this to shop and to sanity-check a quote, then have a licensed pump professional size and select the actual unit for your well.