Well Water Pressure Tanks

Well Water Pressure Tank Sizes: How to Choose the Right One

Pressure tank size is set by one number, and it is not the number of bedrooms. It is how many gallons a minute your pump puts out. Here is how to read that number off your pump, Amtrol's own sizing chart for every Well-X-Trol model, and the honest answer to "should I just go bigger" from someone who installed these for 32 years. In most cases, no.

What size pressure tank do I need for a 3 bedroom house?

For a typical 3 bedroom, 2 bathroom house on a 1/2 HP submersible pump, the right pressure tank is a 32 gallon WX-203 if the pump is a 7 GPM model, and a 34 gallon WX-205 or 44 gallon WX-250 if it is a 10 to 12 GPM model. That is Amtrol's own chart, and it matches what I tell callers. When a homeowner with a three bedroom house asked me this summer whether he needed something bigger, the answer was "a WX-203 Pro Series would be plenty."

If you cannot find the GPM and want one answer, order the WX-250. On Amtrol's chart it is the correct tank for every 1/2 HP pump from 7 to 12 GPM at 30/50 and for 7 to 10 GPM at 40/60, so it cannot come up short for a 3 bedroom house. The only thing you risk is about $900 you did not need to spend if the pump turns out to be a 7 GPM. If you would rather not risk that, the pump label is a 60 second look, covered in the next section.

The house size only matters because it hints at what pump the well driller installed. Most 2 to 4 bathroom homes got a 1/2 HP pump in the 7 to 12 GPM range. Some got a 3/4 HP or 1 HP because the well is deep, and those pumps often move 10 to 15 GPM, which pushes you to the 44 gallon WX-250 (12 GPM) or a 62 gallon WX-251 (15 GPM, a supply house item we do not stock) for a one-minute run, and to 81 gallon and larger supply house tanks if the motor is 1 HP and you follow Amtrol's two-minute rule for it strictly. So:

Two things the table cannot know. If your pump is over 3/4 HP (1 HP and up), Amtrol wants two minutes of run time, which moves you one size past the right-hand model in each row (the sizing chart below has the exact columns). And if the label on the pump says something different from what "most of them have," believe the label. That is the next section.

Find your pump's GPM in 60 seconds

A submersible pump's model number starts with its nominal flow series in gallons per minute and ends with its horsepower: Goulds 10GS05 is a 10 GPM, 1/2 HP pump; Franklin 7FR07 is a 7 GPM, 3/4 HP pump. The horsepower codes are 05 for 1/2 and 07 for 3/4 on both brands; from 1 HP up Goulds writes 10, 15, 20 and Franklin writes 1, 15, 2. Horsepower (with the stage count) is about how much lift the pump has for a deeper well, not how much water it delivers. A 10 GPM series pump is sold in 1/2, 3/4, 1 and 1-1/2 HP for deeper and deeper wells, and all four are still 10 GPM pumps.

You will not see the pump itself; it is a few hundred feet down the well. What you can find:

• The well driller's report or the pump invoice, usually in the closing paperwork from when you bought the house. The model number is on it.
• The control box (3-wire pumps only), a gray box on the wall near the pressure tank. Its label gives horsepower and often the pump model.
• The old tank. Whoever sized the last one usually matched the pump. If it says WX-203 or "32 gallon" and the pump has not changed, that is your starting point, unless it short cycled its whole life.
• A bucket and a stopwatch. With the pump running and every fixture off, open the boiler drain on the tank tee into a 5 gallon bucket after the tank has emptied. Gallons divided by minutes is your real flow. Rough, but it beats guessing.

If none of that works, text me a photo of the control box or the old tank's label and I will tell you the size. That is a two minute answer for me and a $400 mistake avoided for you.

Ask Aidan

Not sure what pump you have?

Text me a photo of the old tank's label, the control box, or the pump paperwork, and how many bathrooms. I'll text back the tank that fits, including if it's one we don't sell.

Text Aidan 443-277-2204 Call800-460-5810Aidan

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Pressure Tank Sizing Chart

Amtrol sizes Well-X-Trol tanks by pump flow and switch setting: at 30/50, a 5 GPM pump takes a WX-202, 7 GPM a WX-202XL, 10 GPM a WX-203, 12 GPM a WX-250 and 15 GPM a WX-251 for a one-minute pump run; double the run time (their recommendation for pumps over 3/4 HP) and each row moves up two to three sizes. This is Amtrol's Quick Sizing Chart (MC 7025, March 2026 edition) exactly as printed, with the models we carry linked and priced; starred models are supply house sizes we do not stock. Where Amtrol lists a size we do not carry, the next stocked size with more usable water is a fair substitute (a WX-203 in place of the 202XL, a WX-250 in place of the 203 at 40/60); a smaller one is not. Prices are our package price with the tank tee kit, switch, gauge, relief valve and freight included.

*Not something we stock. The WX-251, 255, 302 and 350 are 62 to 119 gallon tanks sold through plumbing supply houses (Ferguson, SupplyHouse and the like) for roughly $900 to $2,400 bare depending on size and dealer. I list them so the chart is complete, not to sell you one. Source: Amtrol Well-X-Trol Quick Sizing Chart, form MC 7025.

Read the chart the way Amtrol intends: pick the column for your switch setting and your pump's horsepower class, then the row for its GPM. The one-minute column is the floor for pumps through 3/4 HP. The two-minute column is what Amtrol recommends once you are over 3/4 HP (1 HP and up), because larger motors want a longer run to cool. It is not a "premium" column for everyone. On a 1/2 HP, 10 GPM pump, the WX-203 is the correctly sized tank (the WX-205 holds a little more in a shorter shell), and the WX-250 is the comfortable one if you have the floor space and want fewer pump starts. Anything larger is money spent on water you will never notice.

Well-X-Trol Pressure Tank Size Chart (every model)

Well-X-Trol residential tanks run from the 14 gallon WX-201 (4.3 gallons of drawdown at 30/50) to the 119 gallon WX-350 (36.8 gallons); the four we sell, the WX-202, 203, 205 and 250, cover 20 to 44 gallons and 6.2 to 13.6 gallons of drawdown, which is the range nearly every house needs. All numbers are from Amtrol's current spec sheet. Drawdown is the water you actually get between pump starts; it is what you size on. Volume is just how big the steel is.

Factory precharge on every model is 38 PSI (set for a 40/60 switch; drop it to 28 for 30/50). Maximum working pressure 150 PSI. Amtrol notes drawdown varies with water temperature and exact switch pressures. Source: Amtrol Well-X-Trol specification sheet, MC 4400.

Notice the WX-205 and the WX-203 give you almost the same water (10.5 vs 9.9 gallons). The 205 is short and wide, 30 inches tall, and it exists for crawl spaces and under-stair closets where a 47 inch WX-203 will not stand up. If you have the height, the 203 is $400 less for the same job. The WX-250 is the biggest tank I put in a normal house.

Why Tank Size Matters

The pressure tank's only job is to hold a few gallons under pressure so the pump does not have to start for a toilet flush or a glass of water; size it too small and the pump starts dozens of extra times a day, which is what wears out a motor and a pressure switch, but size it larger than the chart and nothing else improves. Your water pressure is set by the pressure switch (30/50 or 40/60), not by the tank. A 119 gallon tank and a 20 gallon tank on the same switch give you exactly the same pressure at the shower head.

Here is what actually happens when you use water. The tank starts full at 50 PSI. You open a shower at 2.5 GPM. The tank's air charge pushes water out and the pressure falls. On a WX-203 it takes about four minutes to fall to 30 PSI, the pump starts, and from that moment on the pump is supplying the shower directly and refilling the tank on the side. It shuts off a minute or so after you close the valve. So a bigger tank changes exactly one thing: how many gallons come out before the pump starts. Once it has started, tank size is irrelevant.

That is why the sizing rule is about the pump, not the house. A 10 GPM pump refilling 10 gallons of drawdown runs for about a minute with no water being used, long enough to cool the motor and keep the switch contacts from arcing on every start. Amtrol's one-minute rule (ESP I) is exactly that, and they extend it to two minutes for motors over 3/4 HP because bigger motors need more cooling. There is no third tier where three or four minutes buys you more pump life.

Aidan on what size pressure tank you need, and why a bigger tank does not mean more pressure (starts at 1:10).

The Drawdown Formula Explained

Drawdown is the water a tank delivers between the pump's cut-in and cut-out pressures; on a Well-X-Trol it is about 31 percent of the tank's volume at 30/50, 27 percent at 40/60, and 24 percent at 50/70, so a 44 gallon WX-250 gives 13.6, 11.8 or 10.4 gallons depending on the switch. The rest of the shell is the compressed air that does the pushing.

The precharge is where sizing gets thrown away. Every Well-X-Trol ships at 38 PSI, which is correct for a 40/60 switch. Put it on a 30/50 system without letting air out to 28, and the tank is fighting the switch: the pump cuts in at 30 while the bladder still wants 38, so you lose a chunk of your drawdown and the pump starts sooner than it should. Check it once with a tire gauge on the stem under the cap on top, pump off, tank drained, before the system goes live. Two PSI below cut-in. Full walkthrough in the troubleshooting guide.

When to Size Up (and when a bigger tank is the wrong fix)

Go one size up for a pump over 3/4 HP, for a house where two showers and a washer genuinely run at once, and for a mechanical room you would rather not visit again for 20 years; do not buy a bigger tank to fix low pressure, a low-yield well, or a backwashing filter, because a pressure tank does none of those jobs.

Reasons to go up one size

• A pump over 3/4 HP (1 HP and up). This is Amtrol's own rule, not mine: motors over 3/4 HP want a two minute run. Use the right-hand columns in the sizing chart.
• Heavy simultaneous use. Two showers, a washer and a dishwasher together can pull 8 to 10 GPM. That does not change the sizing rule (the pump is running the whole time anyway), but a 44 gallon tank on a 10 GPM pump gives you a few more minutes of quiet before it starts, which people with a bedroom above the mechanical room appreciate.
• You are replacing a tank that short cycled its whole life. If the old one was a 20 gallon on a 10 GPM pump, do not match it. Size from the pump.
• Resale. If you are going to sell the house in a few years, the 44 gallon tank is a line item the inspector writes down. That is the one case where I tell people to spend the extra money on size.

Problems a bigger tank will not fix

• Low water pressure. Pressure is the switch. If you want stronger showers, move the switch from 30/50 to 40/60 (and the precharge from 28 to 38). Past 60 PSI you start stressing old plumbing; if you want rock-steady pressure at any flow, that is a constant pressure system, not a tank.
• A low-yield well. A pressure tank is not a storage tank. If your well makes 3 gallons a minute and the house wants 8, the tank runs dry in a couple of minutes no matter how big it is, and then you are running the pump on the well's 3 GPM. The fix is a cistern and a second pump, or lower-flow fixtures. Not a WX-350.
• A backwashing iron filter or softener. A filter backwashes at 5 to 10 GPM for 10 to 15 minutes, which is far more than any tank's drawdown; the pump supplies the backwash directly, the tank just decides how many seconds pass before it starts. What matters is that your pump can deliver the filter's backwash rate. That is a pump and filter question, and if you are choosing a filter, check the backwash rate on the filter you are looking at against your pump's GPM before you buy. The iron filter collection is the place to start.
• A pump that starts every time you flush. That is normal. A toilet uses 1.6 gallons; the pump starts when the drawdown is gone. Someone with a WX-202 (6.2 gallons) who flushes twice and runs the sink will hear the pump, and nothing is wrong. See the what normal looks like before you spend anything.

One more, because I hear it every month: two tanks. A well company tells a homeowner that plumbing two tanks side by side will stop the pump cycling. Amtrol does list doubled tanks in their chart, for 20 GPM and larger pumps at 40/60 that a single WX-255 cannot cover. For a house on a 10 GPM pump it is a way to sell a second tank. If two tanks were the answer for normal homes, every basement would have two, and almost none do.

Common Sizing Mistakes

1. Reading the gallon rating as water. The 44 on a WX-250 is the shell. You get 13.6 gallons at 30/50. Every chart above uses drawdown for a reason.
2. Sizing from bedrooms instead of the pump. Bedrooms are what the internet asks about. GPM is what the tank cares about. Go read the label.
3. Buying up to "fix" pressure, cycling, or a weak well. Covered above. Each has a real fix, and none of them is a bigger tank.
4. Leaving the precharge at 38 on a 30/50 system. Costs you drawdown you paid for and makes the pump start early. Two PSI below cut-in, checked with a tire gauge.
5. Matching the old tank without asking whether it was ever right. If the pump was upsized when the well was deepened, or the old tank short cycled from day one, the old size is not evidence.
6. Buying the biggest tank that fits. A caller asked me whether going to a bigger tank would make his pump cycle less. My answer, word for word: I would not go any bigger than a WX-250, and past that a bigger tank is honestly a waste of money, because once the drawdown is gone you are running off the pump anyway.

Frequently Asked Questions

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