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Quick answer from a licensed plumber: A tank’s gallon size is not the water you get between pump starts. Amtrol’s chart rates its 20-gallon WX-202 at 6.2 gallons of drawdown at 30/50, 5.4 at 40/60 and 4.7 at 50/70. To size one, Amtrol uses pump flow and a minimum run time (1 minute up to 3/4 hp, 2 minutes above). A 10 gpm, 3/4 hp pump at 30/50 gets a WX-203 (32 gal); at 40/60, a WX-250 (44 gal).
Amtrol Well-X-Trol Drawdown Chart
Drawdown is the water you can draw before the pump restarts. Amtrol defines it as “The amount of water delivered between pump cycles.” It is always a fraction of the tank’s gallon size, and it shrinks as the pressure setting goes up. Straight from Amtrol’s Quick Sizing Guide:
| Model | Tank volume (gal) | Drawdown 30/50 | Drawdown 40/60 | Drawdown 50/70 | Diameter x height (in.) | Connection |
|---|---|---|---|---|---|---|
| WX-101 | 2.0 | 0.6 | 0.6 | 0.5 | 8 x 13 | 3/4″ NPTM |
| WX-102 | 4.4 | 1.4 | 1.2 | 1.0 | 11 x 15 | 3/4″ NPTM |
| WX-103 | 6.7 | 2.1 | 1.8 | 1.6 | 11 x 20 | 3/4″ NPTM |
| WX-104 | 10.3 | 3.2 | 2.8 | 2.4 | 15 x 18 | 1″ NPTM |
| WX-200 | 14.0 | 4.3 | 3.8 | 3.3 | 15 x 22 | 1″ NPTM |
| WX-201 | 14.0 | 4.3 | 3.8 | 3.3 | 15 x 25 | 1″ NPTF |
| WX-202 | 20.0 | 6.2 | 5.4 | 4.7 | 15 x 32 | 1″ NPTF |
| WX-202XL | 26.0 | 8.0 | 7.0 | 6.1 | 15 x 39 | 1″ NPTF |
| WX-203 | 32.0 | 9.9 | 8.6 | 7.6 | 15 x 47 | 1″ NPTF |
| WX-205 | 34.0 | 10.5 | 9.1 | 8.0 | 22 x 30 | 1-1/4″ NPTF |
| WX-250 | 44.0 | 13.6 | 11.8 | 10.4 | 22 x 36 | 1-1/4″ NPTF |
| WX-251 | 62.0 | 19.2 | 16.6 | 14.6 | 22 x 47 | 1-1/4″ NPTF |
| WX-255 | 81.0 | 25.0 | 21.7 | 19.1 | 22 x 57 | 1-1/4″ NPTF |
| WX-302 | 86.0 | 26.6 | 23.0 | 20.3 | 26 x 47 | 1-1/4″ NPTF |
| WX-350 | 119.0 | 36.8 | 31.9 | 28.1 | 26 x 62 | 1-1/4″ NPTF |
Amtrol adds a caution under the table: “Drawdown can be affected by various ambient and system conditions, including temperature and pressure.” Treat the numbers as the maker’s rating, not a guarantee for your basement.
One pattern is worth knowing (our arithmetic on Amtrol’s figures): at 30/50, every Well-X-Trol from the WX-102 up delivers about 31 percent of its volume per cycle (30.7 to 31.8 percent). A 20-gallon tank gives about 6 gallons, not 20. To compare 20-gallon tanks across makers, see our well pressure tank picks.
Amtrol’s Quick Sizing Table
Amtrol sizes a tank from the pump’s flow rate and a minimum run time: “Pumps Up To 3/4 hp & 1 Minute Run Time” and “Pumps Over 3/4 hp & 2 Minute Run Time.” Its recommendations, by pump flow in gallons per minute (for real flow numbers, see our shallow well pump comparison or, for deeper wells, our submersible well pump picks):
| Pump flow (approx. gpm) | Up to 3/4 hp: 30/50 | Up to 3/4 hp: 40/60 | Up to 3/4 hp: 50/70 | Over 3/4 hp: 30/50 | Over 3/4 hp: 40/60 | Over 3/4 hp: 50/70 |
|---|---|---|---|---|---|---|
| 5 | WX-202 | WX-202 | WX-202XL | WX-205 | WX-250 | WX-250 |
| 7 | WX-202XL | WX-203 | WX-203 | WX-251 | WX-251 | WX-251 |
| 10 | WX-203 | WX-250 | WX-250 | WX-255 | WX-255 | WX-302 |
| 12 | WX-250 | WX-251 | WX-251 | WX-255 | WX-350 | WX-350 |
| 15 | WX-251 | WX-251 | WX-255 | WX-350 | WX-350 | WX-255 (2) |
| 20 | WX-255 | WX-255 | WX-302 | WX-255 (2) | WX-255 (2) | WX-302 (2) |
| 25 | WX-255 | WX-350 | WX-350 | WX-255 (2) | WX-350 (2) | WX-350 (2) |
| 30 | WX-350 | WX-350 | WX-255 (2) | WX-350 | WX-350 (2) | WX-302 (3) |
| 35 | WX-350 | WX-255 (2) | WX-255 (2) | WX-350 (2) | WX-350 (3) | WX-350 (3) |
| 40 | WX-255 (2) | WX-255 (2) | WX-302 (2) | WX-350 (3) | WX-350 (3) | WX-350 (3) |
A number in parentheses is how many of that tank Amtrol calls for. The 30-gpm row, over 3/4 hp, reads WX-350 at 30/50 but two WX-350s at 40/60; that is how Amtrol prints it.
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How to Use the Table
- Find the pump’s flow rate and horsepower. They are on the pump label or in its manual; the contractor who installed the pump will also have them.
- Read the pressure setting on the pressure switch: 30/50, 40/60 or 50/70. Our 30/50 vs 40/60 guide covers what changes between them.
- Read across to the column for your horsepower and setting. Round the flow up to the next row.
- Check the space. Use the diameter and height in the first table: a WX-250 is about 22 by 36 inches. To identify a tank that is already installed, see our Amtrol well tank model number guide.
Replacing an Old Air-Over-Water Tank
Older wells often have a plain steel tank with an air volume control instead of a diaphragm. Wayne’s well pump manual explains the difference: in a conventional tank, the drawdown is “approximately 1/6 of the tanks total volume when operated on a 30/50 pressure setting,” and “a 20 gallon pre-charged tank will have the same usable water or draw-down capacity as a 40 gallon conventional tank.” A diaphragm tank can be about half the size of the tank it replaces and deliver the same water per cycle.
If the Pressure Setting Changes Too
A tank sized for 30/50 delivers less water at 40/60, so the switch and the tank are decided together. If the switch is being replaced at the same time, these two were in stock on October 3, 2026. The Square D 9013FSG2J21 (30/50) and FSG2J24CP (40/60) we used to link were sold only by third-party sellers on 10/3/2026, so they are not linked.
Match the setting to the column you sized the tank from:
- 30/50: Eastman 60043 pressure switch, sold by Amazon; the listing gives it a 30-50 psi factory preset and a 1/4-inch FPT connection, for jet, submersible and reciprocating pumps.
- 40/60: Merrill MPSM44060 pressure switch, sold on Amazon by Merrill Manufacturing; the listing gives a 40 psi cut-in, a 60 psi cut-off and a low-pressure cut-off with a manual lever.
FAQ
How much water does a 20 gallon well tank hold?
Far less than 20 gallons per cycle. Amtrol rates its 20-gallon WX-202 at 6.2 gallons of drawdown at 30/50, 5.4 at 40/60 and 4.7 at 50/70.
What size pressure tank do I need for a 10 gpm well pump?
On Amtrol’s quick sizing table, a 10 gpm pump up to 3/4 hp calls for a WX-203 at 30/50 or a WX-250 at 40/60 or 50/70. A pump over 3/4 hp at 10 gpm calls for a WX-255 at 30/50 or 40/60 and a WX-302 at 50/70.
What is drawdown on a well pressure tank?
The amount of water the tank delivers between pump cycles. Amtrol says the larger the tank, the greater the drawdown and the less the pump needs to run.
Does a bigger pressure tank mean better water pressure?
No. The pressure switch sets the pressure. A bigger tank gives more water per cycle, so the pump starts less often; Amtrol says larger tank sizes also “provide more consistent water pressure.”
What run time does Amtrol size well tanks for?
One minute of pump run time for pumps up to 3/4 hp and two minutes for pumps over 3/4 hp, per its Well-X-Trol quick sizing table.
Why Trust Plumbing Picks
Plumbing Picks is written by a licensed Georgia journeyman plumber. Every part number, size, setting and fit on this page comes from the manufacturer pages and documents listed below, which we opened and read; product details come from those makers and from the Amazon listings, not from hands-on testing. Each linked product was new and in stock, sold by Amazon or by the maker’s own store, when we checked on October 3, 2026. Where a link goes to Amazon we may earn a commission at no extra cost to you. Nobody pays to be included here. More on who writes this and how we pick.
Sources
- Amtrol: Well-X-Trol Quick Sizing Guide (MC7025, 03/26)
- Amtrol: Well-X-Trol WX-100, 200 and 300 submittal (MC4400)
- Amtrol: Well-X-Trol installation and operation instructions (MC10185)
- Amtrol: Well-X-Trol well tanks
- Wayne: CWS Series convertible well jet pump manual (640003W-001 E)
Bottom Line
Size a well tank by drawdown, not gallons. Find the pump’s flow and horsepower, read the pressure setting, and use Amtrol’s table: about 31 percent of a Well-X-Trol’s volume comes out per cycle at 30/50, less at higher settings. Installation, the relief valve and the precharge are work for a licensed professional.