The fixture-count method
Residential well pumps are sized by peak demand, estimated at 1 GPM per water fixture: every sink, toilet, shower, dishwasher, washer and hose bib counts one. A 2-bath home with kitchen, laundry and outdoor spigots lands around 9–11 GPM, which is why 10 GPM half-HP to 1-HP submersibles are the standard residential pumps. Pump horsepower is then chosen from the manufacturer's curve for your GPM at your well depth (total dynamic head) — deeper wells need more HP for the same flow.
Don't out-pump your well
The pump must never exceed the well's tested yield. If the well produces 5 GPM but the house demands 10, the fix is not a bigger pump (it would suck the well dry and burn out) but a larger pressure/storage tank buffering peak use. Your well driller's report lists the yield — check it before buying anything.
Pressure tank drawdown
The pressure tank must let the pump rest: minimum 1 gallon of drawdown per GPM of pump capacity (2 is gentler). Drawdown is the usable water between cut-in and cut-out pressure — at the common 40/60 psi setting, a tank's drawdown is roughly 30% of its nominal size. A 10 GPM pump therefore wants a tank with 10–20 gallons of drawdown, i.e. a 32–62 gallon nominal tank.
Frequently asked questions
How many GPM do I need for my house?
Count fixtures (1 GPM each): a typical 2-bathroom home totals 9–11 GPM. Minimum recommended pump size for any full household is about 6 GPM.
What size pressure tank for a 10 GPM pump?
At least 10 gallons of drawdown — about a 32–36 gallon nominal tank at 40/60 psi. Doubling drawdown halves pump cycling and extends pump life.
What HP well pump do I need?
GPM decides demand; horsepower depends on well depth. As a rough guide at 10 GPM: ~½ HP to 150 ft, ¾ HP to 250 ft, 1 HP to 400 ft. Always confirm on the pump curve.
My well only makes 4 GPM — what now?
Size the pump to the well (never above yield) and add storage: a large pressure tank or a cistern with booster pump buffers showers and laundry peaks.