Well Sizer

Methodology and sources

Every figure on this site comes from a published manufacturer document, a university extension or health department page, or a state rule, read on 2026-10-20. The calculators apply two formulas — Boyle's law and the head sum — to those figures. Nothing is estimated.

Pressure tanks

Usable share (drawdown factor). A precharged tank holds a fixed amount of air. By Boyle's law, at gauge pressure P the air takes up (precharge + 14.7) ÷ (P + 14.7) of the tank, so the water drawn off between cut-out and cut-in is

drawdown = volume × (precharge + 14.7) × (1 ÷ (cut-in + 14.7) − 1 ÷ (cut-out + 14.7))

The precharge is set 2 psi below cut-in, as AMTROL instructs: "Using an air gauge, adjust the precharge to 2 psig below the cut-in (LO) setting." (AMTROL Well-X1 Installation & Operating Instructions).

Check against the manufacturer. With the precharge equal to cut-in the formula reduces to 1 − (cut-in + 14.7) ÷ (cut-out + 14.7), which is how Goulds' Table 2 pressure factors are built; it reproduces 149 of the table's 152 cells to within 0.01. Goulds' Table 1 tank drawdowns, which count water down to a precharge 2 psi under cut-in, are reproduced for every tank except one cell (V250 at 38/60: 25.6 printed, 24.6 by the law, while the larger V260 shows 25.0). This site sizes between the switch points, the water the pump actually refills, so its drawdowns are a little smaller than manufacturer tables and its tanks a little larger.

Run time. Franklin Electric: "Run motors a minimum of one minute to dissipate heat build up from starting current." Tank volume = pump gpm × run minutes ÷ usable share. Starts per 24 hours (single-phase): up to 0.75 hp, 300; 1 to 5.5 hp, 100; 7.5 to 30 hp, 50. Source: Franklin Electric, Application • Installation • Maintenance (AIM) Manual, Submersible Systems, 60 Hz (2026) — copy published by the Nebraska Department of Water, Environment and Energy.

Tank models. Goulds HydroPro total volumes, Table 1 of Goulds Water Technology (Xylem), Technical Manual TTECHWP R7. Drawdown based on a 22 psi differential and Boyle's Law. Temperature, elevation and pressure can all affect drawdown volume.

Pump flow and head

Demand. Goulds' private-residence table: a seven-minute peak of 45 gallons (1 bath, 7 gpm pump), 70 gallons (1½ bath, 10 gpm pump), 98 gallons (2–2½ bath, 14 gpm pump), 122 gallons (3–4 bath, 17 gpm pump). "Normal seven minute peak demand (gallons). Peak demand can occur several times during morning and evening hours." Penn State Extension's table by bedrooms and bathrooms (source) is shown for comparison: "For most single-family homes, a minimum flow of 6 GPM is suggested from a well or spring." New York State Department of Health: "A well for a private residence should, if possible, be constructed to produce a minimum long-term sustainable yield of 5 gpm or more." (fact sheet).

Total dynamic head = pumping water level + rise from wellhead to tank + cut-out pressure × 2.31 + pipe friction. Goulds: "H = 2.31 x psi". Friction comes from Goulds' "Sch 40 plastic pipe: friction loss (in feet of head) per 100 ft" table, interpolated between its listed flows, for 1, 1¼, 1½ and 2-inch pipe.

Pump selection. The ratings are Goulds' 10GS selection chart (Model 10GS selection chart — horsepower range ½–3, recommended range 3–16 GPM, 60 Hz, 3450 RPM): gallons per minute by "Depth to Water in Feet/Ratings in GPM (Gallons per Minute)" and pressure, from Goulds Water Technology, Model GS 5–25 GPM technical brochure B5-25GS (March 2012) — copy hosted by a retailer; the current Xylem brochure has no ratings tables. The chart does not state a pipe-friction allowance, so the calculator adds the rise to the tank and the pipe friction to the depth before reading it, then reads the next listed depth and pressure at or above the result. The pick is the smallest model that delivers the needed flow at the cut-in pressure and is still rated at the cut-out pressure — an editorial rule, stated so you can apply your own.

The chart was extracted by column position from the PDF and checked against the rendered page; every value sat within 1 point of its column.

Septic tanks

Each state's minimum septic tank liquid capacity for a single-family home, read from the state's own rule (administrative code, permit or agency standard) on the check date. Where a rule gives a formula or a per-bedroom increment instead of a printed row, the bedroom figures are that arithmetic, and the state page says so. Wisconsin is left out: its code sets design flow but no tank volume, which comes from state-approved component manuals.

Not listed yet: Wisconsin. Wisconsin's code sets a design flow of 150 gallons a day per bedroom but no tank volume; tanks are sized under state-approved component manuals, which are not published as a single table. Alabama's table is published only as an image and was read from the rendered page. Arkansas prints its four-bedroom row as "3"; the site reads it with the rule's +250-per-bedroom note.

What this site does not do

Corrections: hello@wellsizer.com. Data version 2026-10-20.