How to Size a Well Pump
Sizing a well pump means balancing household demand against the well’s tested yield, depth, and pressure needs — not just copying the old horsepower.
Quick Answer
A well pump is sized by matching the flow and lift your household needs against your well’s tested yield, depth, and target pressure. Bigger is not better: an oversized pump can out-run a low-yield well, pull in sand and air, and short cycle. The pump must be matched to the manufacturer’s pump curve and your well’s measured yield, which is why a professional sizing is important.
What goes into sizing
- Peak household demand: roughly how many gallons per minute you need when multiple fixtures run.
- Well yield: the gallons per minute the well can actually sustain, from a well-yield (draw-down) test.
- Total lift and head: the vertical distance to raise water plus friction losses. Remember 1 psi ≈ 2.31 ft of head.
- Target pressure: the cut-in/cut-out range you want to deliver (for example 40/60 psi).
- Pump curve: the manufacturer’s chart showing flow at a given head — the real basis for matching.
A simple way to estimate demand
Rough demand estimate
- 1List the fixtures likely to run at the same time during a busy period.
- 2Add up a reasonable flow for each to get a peak gallons-per-minute figure.
- 3Compare that peak to the well’s tested yield — the pump cannot sustainably deliver more than the well produces.
- 4Note the total lift and target pressure so a pro can read the right point on the pump curve.
Do not oversize
Why the pump curve matters
Two pumps with the same horsepower can deliver very different flows at the same depth. The pump curve shows how flow falls as head (lift) rises. Sizing by horsepower alone, or by copying the old unit, often misses the mark — the curve and the well’s tested yield are what matter.
Final sizing is a professional step
Frequently asked questions
- Can I just buy the same horsepower as my old pump?
- Not reliably. If the old pump was wrong for the well, you would repeat the mistake. Size to the well’s tested yield, required head, and the manufacturer’s pump curve instead.
- How do I know my well’s yield?
- A draw-down or well-yield test, usually run by a well contractor, measures how many gallons per minute the well sustains. It is the ceiling for how much the pump can deliver long-term.
Sources & references
- Protect Your Home’s Water: Private Wells — US EPA, accessed October 29, 2026
This guide is documentary research and general information, not a laboratory analysis or professional assessment of your specific well. See our disclaimer.