Well pressure tank sizing: 20–86 gallons for a 5–15 gpm pump

Size a well pressure tank so the pump runs at least one minute per cycle. On Amtrol's chart at a 30/50 switch, a 5 gpm pump needs a 20 gallon tank and a 10 gpm pump a 34 gallon tank, or 62 gallons for a two minute run. Bare tanks in that range sell for $340–$1,150.

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Key figures

Item	Figure
Minimum pump run time per cycle	1 minute^3,4
Drawdown multiplier at a 30/50 switch (Usable water is about 29% of tank volume)	0.29³
10 gpm pump, one minute run	34 gallon tank¹
Bare 20 gallon tank (WX-202)	$340–$370²
Bare 44 gallon tank (WX-250)	$700–$1,150²

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The sizing rule

Pentair's Wellmate data sheet gives the formula: pump flow in gallons per minute, times the minimum run time, divided by the drawdown multiplier for your switch setting, equals the tank size. Wellmate recommends at least one minute of run time. A trade article in The Driller puts it the same way: a 10 gpm pump needs a tank that holds about 10 gallons of drawdown, the water the tank delivers between the pump shutting off and starting again.

Drawdown is a fraction of the tank, not the whole thing. A tank labeled 20 gallons holds far less usable water than that, which is why the multiplier matters.

Sources^3,4

Drawdown by switch setting

The higher and narrower the pressure band, the less water each gallon of tank delivers. These are Wellmate's multipliers.

Drawdown multiplier by pressure switch setting
Switch (start / stop psi)	Multiplier
20 / 40	0.35
30 / 50	0.29
40 / 60	0.25
20 / 50	0.42
30 / 60	0.37

Sources³

Tank size by pump flow

Amtrol sizes its Well-X-Trol line two ways. Its one minute chart (ESP I) is meant for pumps up to 3/4 hp, and its two minute chart (ESP II) for pumps of 3/4 hp or larger. The table reads Amtrol's quick sizing figures at a 30/50 switch.

Amtrol tank by pump flow at a 30/50 psi switch
Pump flow	One minute run	Two minute run
5 gpm	20 gal (WX-202)	not listed
10 gpm	34 gal (WX-205)	62 gal (WX-251)
15 gpm	44 gal (WX-250)	86 gal (WX-302)

A bigger tank costs more up front. Wellmate notes that it also means fewer pump starts and longer runs, which adds to pump life.

Sources^1,3

What the tanks cost

Mid Atlantic Water, a supplier, lists bare Well-X-Trol tanks at supply houses at about $340–$370 for the 20 gallon WX-202, $500–$600 for the 32 gallon WX-203 and $700–$1,150 for the 44 gallon WX-250. Box-store steel tanks run $200–$550 across 20 to 44 gallons. A plumber who supplies and installs the tank charges $800–$1,800 all-in.

Sources²

Common questions

Can a pressure tank be too big?

Not for the pump. Wellmate says a larger tank means fewer cycles and longer run times, which adds to pump life. The limits are cost and floor space.

What size tank does a 1 hp pump need?

Amtrol recommends sizing pumps of 3/4 hp or larger on its two minute chart. At 10 gpm and a 30/50 switch, that is a 62 gallon tank.

Why does a 20 gallon tank only give me about 6 gallons?

Drawdown is a fraction of the tank. Amtrol lists the 20 gallon WX-202 at 6.2 gallons of drawdown at a 30/50 switch.

For the cost of a finished well in your state, see how much a well costs.

Sources

Well-X-Trol WX-101 to WX-350 specifications and Quick Sizing Chart (MC 7025 04/14) , Amtrol Inc.. Accessed November 9, 2026. 04/2014
How Much Does a Well Pressure Tank Cost? , Mid Atlantic Water LLC (water-system supplier, nationwide). Accessed November 8, 2026. Updated 2026-10-14; prices live as of 2026-09. Tank replaced by plumber $800-$1,800 all-in (20-44 gal); labor $200-$500; submersible pump job $1,000-$2,500; tank plus pump $2,500-$5,000+. Supplier, not an installer.
WELLMATE WM01, WM02 In-Line Pressure Tanks datasheet , Pentair (Wellmate). Accessed November 9, 2026. PDF metadata 2012-12-06 (file revision mr14)
Pressure Tanks , The Driller (Bob Pelikan). Accessed November 9, 2026. January 3, 2006

How these figures are built: methodology. Updated November 2026.