How it works
1. Flow: how many gallons per minute
Goulds Water Technology (Xylem) publishes a household peak-demand table by bathrooms: 1 bath = 7 GPM, 1½ = 10, 2–2½ = 14, 3–4 = 17 GPM. An alternative rule in the same manual is 1 GPM per water fixture, counting every faucet, toilet, shower, appliance and outside hose bib. MU Extension guide G1801 gives a bedroom × bathroom table (for example, 4 bedrooms and 2 baths = 14 GPM).
2. Head: how hard the pump has to push
TDH (ft) = pumping water level + vertical rise + (pressure in psi × 2.31) + friction loss
The pumping level is where the water sits while the pump runs, which is lower than the static level. Each psi of pressure at the tank equals 2.31 ft of head, so a 60 psi cut-out adds about 139 ft.
3. Horsepower estimate
Brake HP = GPM × TDH ÷ (3,960 × pump efficiency)
3,960 converts gallons-per-minute × feet of water into horsepower. Efficiency comes from the pump curve at your duty point, so treat the HP figure as a cross-check. Always choose the pump from the manufacturer curve that meets your GPM at your TDH.
Worked example
A house with 2 bathrooms needs 14 GPM. The pumping level is 150 ft, the tank sits 20 ft above the well head, the switch cuts out at 60 psi and the friction loss is about 15 ft:
TDH = 150 + 20 + 60 × 2.31 + 15 = 150 + 20 + 138.6 + 15 = 323.6 ft
HP = 14 × 323.6 ÷ (3,960 × 0.60) ≈ 1.91 hp
So you would look for a pump curve that delivers about 14 GPM at about 324 ft of head.
Assumptions and limits
- The pump cannot deliver more than the well yields. Get the well's tested yield from the driller's log; if peak demand exceeds it, add storage.
- Friction loss depends on pipe size, material, length and fittings. Use a friction-loss table for your actual pipe.
- Pump efficiency varies along the curve; 60% is only a placeholder.
- Electrical sizing (wire, breaker, control box) is not covered. Use the wire size calculator and the pump manual.