Pressure Tank Size and Drawdown Calculator

A well pressure tank is sized so that the water it gives out between pump stop and pump start keeps the pump running for its minimum time per start. A 10 gpm pump that should run at least 1 minute needs about 10 gallons of drawdown. On a 40/60 pressure switch with the precharge at 38 psi, only about 26% of a tank’s total volume is drawdown, so the tank needs a total volume of about 38.8 gallons: in makers’ common sizes, a 44-gallon tank. The calculator on this page runs the same arithmetic on your pump, your switch and your pump maker’s run time. It is one of our guides to pressure tanks and switches, and it gives an estimate, not a tank choice.

General information, not professional advice. Your pump and tank makers’ manuals and a licensed well or pump contractor decide what’s safe for your system; your state or county decides what needs a permit or a licence.

What the calculator works out

A bladder or diaphragm tank holds a cushion of air above (or around) the water. When the pump runs, water squeezes the air until the pressure reaches the switch’s cut-out and the pump stops. As you use water, the air pushes it out until the pressure falls to the cut-in and the pump starts again. The water delivered between those two moments is the drawdown. It is always a fraction of the tank’s total volume, the gallons printed on its label. The precharge is the air pressure in the empty tank. Our explainer on how a pressure tank works covers the parts.

The inputs:

• Pump flow (gpm). The rated flow on the pump label or paperwork is a starting point. A pump delivers less as pressure rises; Amtrol’s sizing sheet averages the flow it reads off the pump curve at cut-in and at cut-out (Amtrol 9017-102 (03/19), Step 2, read 11 Nov 2026). If you only know what reaches a tap, the bucket-test flow calculator measures that, which is the pump’s delivery at that tap, not the well’s yield.
• Switch setting. 20/40, 30/50, 40/60, 50/70 psi, or your own cut-in and cut-out. It’s usually printed on the switch or its box.
• Minimum run time per start. Default 1 minute, from Franklin Electric’s motor manual; 2 minutes for larger pumps by Goulds’ and Amtrol’s sizing rules (below). Your pump maker’s figure decides.
• Precharge below cut-in. Default 2 psi, as Goulds and Amtrol print it (30/50 switch: 28 psi) (Goulds IM096 Rev 14 (2025-10), p. 3; Amtrol 9015-939, p. 4). Some tanks differ: Goulds’ tank owner’s manual says 2-3 psi (IM251R01, p. 5), and Pentair’s instructions for its WellMate UT, HP and SP tanks say 4 psi, or 10 psi for high flows, pressures or temperatures (W3720 Rev P (2014), p. 1, read 11 Nov 2026). Your tank’s own label or manual decides.
• Optional: your current tank’s total volume, the water you use a day, and the motor’s horsepower, to check run time and starts a day.

The formula, step by step

Air in a closed tank follows Boyle’s law: pressure times volume stays the same, as long as you use absolute pressure (gauge reading plus 14.7 psi at sea level). Amtrol’s sizing sheet uses the same 14.7 (9017-102, Step 3), and Goulds and Pentair say their drawdown tables are based on Boyle’s law (Goulds GSSERVICE R10 (8/2025), p. 61; Pentair WM900 (09-01-23), p. 4).

1. Drawdown needed = pump flow × minimum run time. 10 gpm × 1 minute = 10 gallons.
2. Share of the tank that is drawdown = (precharge + 14.7) ÷ (cut-in + 14.7) − (precharge + 14.7) ÷ (cut-out + 14.7). The first term is the share of the tank filled with air when the pump starts; the second, when it stops. The difference is the water given out in between.
3. Minimum total tank volume = drawdown needed ÷ share. This is the same division Amtrol’s sheet uses (“total volume = run-time volume ÷ acceptance factor”, 9017-102, Step 5).
4. Size class: the next size up among the total volumes Amtrol lists from 20 gallons up: 20, 26, 32, 34, 44, 62, 81, 86 or 119 gallons (MC7025 (03/26), p. 1; Flexcon’s chart and Goulds’ table list sizes close to these). Above 119 gallons, Amtrol’s quick-sizing table lists two or three tanks (MC7025, p. 2), and its sizing sheet for larger systems lists bigger single tanks (9017-102, Table 1).

With your own tank entered, the tool turns it round: your tank’s drawdown = total volume × share, and run time per start = that drawdown ÷ pump flow. With your daily use entered, it divides the gallons a day by the drawdown to give a rough count of starts, and compares that count with Franklin’s limit for the motor size you picked.

Worked example

General information, not professional advice. Your pump and tank makers’ manuals and a licensed well or pump contractor decide what’s safe for your system; your state or county decides what needs a permit or a licence.

The tool’s default numbers: a 10 gpm pump, a 40/60 switch, precharge 38 psi, 1 minute minimum run time.

Step	Working	Result
Absolute pressures	38 + 14.7; 40 + 14.7; 60 + 14.7	52.7, 54.7, 74.7 psi
Air share at pump start	52.7 ÷ 54.7	0.9634
Air share at pump stop	52.7 ÷ 74.7	0.7055
Drawdown share	0.9634 − 0.7055	0.2579 (25.79%)
Drawdown needed	10 gpm × 1 min	10.00 gal
Minimum total volume	10 ÷ 0.2579	38.77 gal
Size class	next listed size at or above 38.77	about 44 gal
Check: a 20-gallon tank	20 × 0.2579 = 5.16 gal; 5.16 ÷ 10	0.52 min, under 1 minute
Check: a 44-gallon tank	44 × 0.2579 = 11.35 gal; 11.35 ÷ 10	1.13 min
Starts a day at 300 gal	300 ÷ 11.35	about 26

Quick table by pump flow

The tool’s results at 1 minute of run time and precharge 2 psi below cut-in. Each cell is the minimum total volume, then the next common size. For 2 minutes, double the minimum volume.

Pump flow	30/50 switch	40/60 switch	50/70 switch
5 gpm	16.9 gal → 20	19.4 gal → 20	21.9 gal → 26
7 gpm	23.7 gal → 26	27.1 gal → 32	30.6 gal → 32
10 gpm	33.9 gal → 34	38.8 gal → 44	43.7 gal → 44
12 gpm	40.6 gal → 44	46.5 gal → 62	52.4 gal → 62
15 gpm	50.8 gal → 62	58.2 gal → 62	65.6 gal → 81
20 gpm	67.7 gal → 81	77.5 gal → 81	87.4 gal → 119

Notice two things. A higher switch setting needs a bigger tank for the same pump, because compressed air gives back a smaller share of the tank between two higher pressures. And the tank doesn’t set your pressure: the switch does. A bigger tank at the same setting means more water between starts, so fewer starts, not stronger flow.

Why makers’ charts print more

Put a tank’s label volume into the tool and you’ll get a smaller drawdown than the maker’s brochure prints. That’s not an error on either side. Makers count drawdown in two other ways, and the tool shows both beside its own figure:

Way of counting	Who prints it	Share at 30/50	Share at 40/60	Share at 50/70
Between pump stop and pump start, precharge 2 psi below cut-in (this tool)	no maker table found	29.53%	25.79%	22.88%
Precharge assumed equal to cut-in	Amtrol (MC7025; 9017-102), Pentair (WM900 drawdown factors)	30.91%	26.77%	23.61%
Down to a precharge 2 psi below cut-in	Flexcon (2019 sizing chart), Goulds (GSSERVICE p. 61)	34.00%	29.45%	25.97%

Checked against the makers’ own tables (read 11 Nov 2026):

• Amtrol’s 20-gallon tank at 30/50: Amtrol prints 6.2 gallons (MC7025, p. 1). Set the tool’s precharge gap to 0 and it gives 6.18. With the 2 psi gap, it gives 5.91.
• Amtrol’s 86-gallon tank at 50/70: Amtrol prints 20.3; the tool with a 0 gap gives 20.31.
• Flexcon’s 20-gallon tank at 30/50: Flexcon prints 6.8, with a footnote that its drawdown assumes a precharge 2 psi below cut-in (Flexcon quick sizing chart, 2019). The tool shows 6.80 on its Flexcon-and-Goulds line. Goulds’ 19.9-gallon tank at 28/50 prints 6.8 too (GSSERVICE, p. 61).
• Pentair’s worked example: 8 gpm × 1 minute ÷ 0.30 = 26.7 gallons (WM900, p. 13), using a rounded factor with the precharge at cut-in; Pentair’s own factor table gives .309 for 30/50 (p. 24). With a 0 gap the tool gives 25.88 (8 ÷ 0.309); with its default 2 psi gap, 27.09.
• Amtrol’s large-pump example: 98 gpm × 2 minutes = 196 gallons; acceptance factor 0.518 at 30/78; 196 ÷ 0.518 = 378 gallons (9017-102, Steps 2-5). The tool with a 0 gap and a custom 30/78 setting gives 378.5.

Why the tool keeps its own way: with the precharge 2 psi below cut-in, as Goulds, Amtrol and Flexcon tell installers to set it, the pump starts while there is still a little water in the tank. The water below cut-in comes out only after the pump has started, so it doesn’t stretch the time between starts. Flexcon’s and Goulds’ figures count it; Amtrol’s assume no gap. The tool counts only the water between pump stop and pump start, so its tank size is the cautious one.

Against Amtrol’s own quick-sizing table for pumps up to ¾ hp at 1 minute (MC7025, p. 2), the quick table above lands on the same tank size in 16 of 18 cells. The two that differ: at 10 gpm and 30/50 Amtrol lists its 32-gallon tank, whose printed 9.9 gallons is just under 10, where the tool goes to 34; and at 20 gpm and 50/70 Amtrol’s 86-gallon tank holds 20.3 gallons by its count but 19.7 by the tool’s, so the tool goes to 119. Amtrol, Flexcon and Goulds all footnote that real drawdown varies with conditions such as temperature and pressure; Flexcon and Goulds add elevation.

One minute or two?

The makers we read don’t agree on one run time, so the tool lets you choose. Each figure below comes from the maker’s own document, read 11 Nov 2026:

Maker and document	What it says
Franklin Electric, AIM Manual, 2026 edition (M1311), PDF p. 6	Run motors at least one minute to get rid of the heat from starting current. Most starts per 24 hours, encapsulated motors: 300 up to 0.75 HP; 100 for 1 to 5.5 HP single-phase (300 three-phase); 50 for 7.5 to 30 HP single-phase. Pick pump, tank and controls to keep starts as low as possible.
Goulds, GSSERVICE R10 (8/2025), p. 61	Size the tank for a minimum run time of 1 minute for ⅓ to 1½ HP pumps, 2 minutes for 2 HP and larger.
Amtrol, MC7025 (03/26), p. 2	Quick sizing at 1 minute for pumps up to ¾ hp, 2 minutes for pumps over ¾ hp.
Flexcon, Steel Well Tanks IOM Rev. 9.21, EN p. 1	A properly sized tank stores at least one gallon of water for every gpm of pump capacity (one minute’s worth).

Franklin’s one minute is a minimum for each start, not a sizing target, and the starts-a-day count in the tool is rough: real use comes in bursts, not evenly through the day. Pumps that start too often wear their controls and motors, which Franklin’s manual names as the reason for the limits. The page on short cycling covers a pump that starts much more often than this.

Getting your own numbers

Claims you may see

• “Precharge should equal cut-in.” Goulds, Amtrol and Flexcon all print 2 psi below cut-in, checked on an empty tank (IM096, p. 3-4; 9015-939, p. 4; IOM Rev. 9.21, EN p. 2). Amtrol’s drawdown table assumes precharge at cut-in only as a way of counting.
• “A bigger tank gives more pressure.” The switch sets the pressure range. A bigger tank at the same setting holds more water between starts.
• “Just add air to a tank that’s lost it.” Amtrol forbids it on a tank that has lost air, shows rust or cycles faster than it did (9015-939, p. 4).
• “Drawdown is the tank’s size.” A tank’s drawdown is roughly a quarter to a third of its total volume at common settings (the shares table above).

Next step

• Write down for the installer: the gauge readings at cut-in and cut-out, the setting printed on the switch, the tank’s label (total volume, maker, age if shown), the pump’s label or paperwork (flow, horsepower), how long the pump runs per start if you’ve noticed, and this tool’s result with the run time you used.
• Ask: what run time the pump maker gives for your pump; the drawdown of the tank they propose at your switch setting, and by which maker’s way of counting; the precharge they’ll set; whether the check valve was looked at.
• What can’t wait: a tank that has lost its air or shows rust (don’t re-pressurise it), a breaker that trips again, a pump that won’t stop, or water around the tank. Turn the pump’s breaker off and call a licensed pump installer. EPA advises testing your water after you replace or repair any part of the well system, or after a flood: see EPA’s private well testing page (read 11 Nov 2026) or your state or county health department.

Sources checked on 11 November 2026. Makers’ documents were read on the makers’ own websites on that date: Franklin Electric AIM Manual 2026 (M1311 06-26); Goulds IM096 Rev 14, IM251R01 and GSSERVICE R10; Amtrol 9015-939 (04/18), 9017-102 (03/19), MC7025 (03/26) and MC10188 (04/24); Flexcon Steel Well Tanks IOM Rev. 9.21 and quick sizing chart (2019); Pentair WM900 (09-01-23) and WellMate W3720 Rev P (2014); Square D 65013-005-39 (Sept 2025); EPA’s private well page on protecting your home’s water. We name makers only as sources; we don’t recommend any tank, pump or brand.