Well pumps · sizing
Sizing a residential well pump by GPM and total head
Horsepower is not a size. Residential well selection starts with two numbers a curve can meet: peak flow (GPM) and total dynamic head (feet). Then you pick SQ, SQE, SP, or SQFlex inside that duty — not the other way around.
1) Data you need from the well
- Casing diameter and total depth
- Static water level and expected (or tested) drawdown / yield
- Current or planned pump setting depth
- Voltage available (115 vs 230 V single-phase is not interchangeable)
- Sand history, iron, H₂S, aggressiveness (affects materials and treatment — not the curve math)
2) Peak flow (GPM)
Add simultaneous fixtures you actually expect: showers, laundry, irrigation zones, livestock troughs, etc. Rule-of-thumb fixture counts are starting points only. Irrigation often dwarfs indoor peaks. Softener backwash and filter flush are intermittent but can be large — see hard-water pages after the well can deliver.
3) Total dynamic head (TDH)
TDH is the sum of:
- Lift — from pumping water level to the point of discharge (include drawdown, not only static level)
- Friction — pipe size, length, fittings, check valves (use a friction chart; undersized drop pipe is a silent head tax)
- Discharge pressure — convert desired PSI to feet (≈ 2.31 ft per psi). Example: 60 psi ≈ 139 ft of head
- Elevation after the wellhead if you push uphill to a tank or house
Constant-pressure SQE systems still need the pump to make setpoint pressure at the required flow. The CU does not remove head — it only varies speed within the curve.
4) Read the curve
- Find models whose published Q/H envelope covers your duty point with margin (wear, slightly more drawdown, future fixtures).
- Stay out of the far-left (deadhead / overheating risk) and far-right (low head, high flow, motor load) abuse zones the literature warns about.
- Match voltage and phase. Confirm cable length voltage drop for deep sets.
5) Then pick the family
| If… | Look at… |
|---|---|
| 3" or restricted casing, grid power, switch/tank OK | SQ |
| Same, but want steadier pressure | SQE + CU |
| 4"+ casing, higher flow/head, conventional service | SP |
| Solar / remote / hybrid energy + storage strategy | SQFlex |
Worked example shape (illustrative only)
Suppose indoor peak ≈ 10 GPM and you want 50 psi at the house gauge. Pumping water level is 120 ft below the gauge elevation, friction ≈ 15 ft, desired pressure ≈ 50 × 2.31 ≈ 116 ft. Rough TDH ≈ 120 + 15 + 116 = 251 ft at 10 GPM. You would then search SQ / SQE / SP curves for a point near 10 GPM @ 251 ft — not “a 1.5 HP pump.” Your numbers will differ; do not reuse 251 ft.
Final pump selection needs your well data and a published performance curve. Talk to a licensed well contractor for design, pull/replace, and code compliance. For product questions, you can also contact R.E. Merrill & Associates.
Hard-water cross-links
- Iron in well water — treatment order after the well can deliver flow
- Rotten-egg / H₂S — smell is not a pump feature
- Size softener by GPG — once pressure and yield are stable