A pressure tank's size depends on your pump's flow rate and how often you want it to cycle

The right pressure tank size keeps your well pump from running constantly and gives you steady water pressure throughout your home. A tank that is too small will cause the pump to start and stop dozens of times a day, wearing it out fast. A tank that is too large wastes space and money. The correct size sits between those extremes and is based on three numbers: your pump's flow rate in gallons per minute (GPM), the pressure range your system uses, and how many times per hour you want the pump to cycle.

Most residential well systems use a pressure range of 30 to 50 pounds per square inch (PSI)—the pump turns on at 30 PSI and shuts off at 50 PSI. Some systems run 40 to 60 PSI instead. The wider your pressure band, the larger your tank can be while still cycling a reasonable number of times. A typical household well pump cycles 4 to 8 times per hour, though some homeowners accept up to 10 cycles per hour to save money on tank size.

Key Takeaways

  • Tank size is calculated from your pump's flow rate in GPM, your pressure range (usually 30–50 PSI), and your target cycle rate (typically 4–8 cycles per hour).
  • You can find your pump's flow rate on the pump nameplate, in your well driller's paperwork, or by timing how long it takes to fill a 5-gallon bucket.
  • The formula is: (GPM × 60 ÷ cycles per hour) × (pressure differential ÷ 100) = tank size in gallons, but online calculators and pump suppliers can do this math for you.
  • A tank that is too small causes the pump to cycle excessively and fail early; a tank that is too large wastes money and space without improving performance.
  • If you are replacing an existing tank, check your current tank's size and cycle rate before ordering—your system may already be sized correctly.

Find your pump's flow rate in gallons per minute

Your pump's flow rate is the amount of water it delivers per minute under normal conditions. This number is printed on the pump's nameplate, a metal tag bolted to the motor or pump housing. Look for "GPM" or "gallons per minute" on that tag. If the nameplate is missing or illegible, check your well driller's paperwork—most drillers keep records of the pump model and flow rate.

If you cannot find the paperwork, you can measure the flow rate yourself. Turn on an outdoor faucet or the main water line and time how long it takes to fill a 5-gallon bucket. Divide 5 by the number of minutes it took, then multiply by 60 to get gallons per minute. For example, if the bucket fills in 30 seconds (0.5 minutes), your flow rate is 5 ÷ 0.5 × 60 = 600 GPM. This method is less precise than the nameplate because water pressure and temperature affect flow, but it gives you a working number.

Determine your pressure range and cycle rate

Your pressure switch controls when the pump turns on and off. Most residential systems are set to 30 PSI on (cut-in) and 50 PSI off (cut-out), giving a 20 PSI differential. Some systems use 40 PSI on and 60 PSI off, also a 20 PSI differential. A few older or specialized systems use a 10 PSI or 30 PSI differential instead. You can find your settings by looking at the pressure switch itself—it usually has two adjustment screws labeled with PSI numbers—or by calling your well contractor.

Your cycle rate is how many times per hour the pump turns on. A rate of 4 to 8 cycles per hour is standard and keeps the pump from wearing out too quickly. Some homeowners accept 10 cycles per hour to use a smaller (cheaper) tank. If you want fewer cycles—say, 3 per hour—you will need a larger tank. The cycle rate you choose is a trade-off between tank cost and pump longevity.

Use the sizing formula or an online calculator

The standard formula for tank sizing is:

(GPM × 60 ÷ cycles per hour) × (pressure differential ÷ 100) = tank size in gallons

Here is a worked example. Suppose your pump delivers 10 GPM, your pressure range is 30 to 50 PSI (a 20 PSI differential), and you want 6 cycles per hour. The math is: (10 × 60 ÷ 6) × (20 ÷ 100) = 100 × 0.2 = 20 gallons. You would need a 20-gallon tank.

If the math feels tedious, most well pump suppliers and tank manufacturers offer online calculators where you enter your GPM, pressure range, and cycle rate, and the calculator returns a tank size. Pentair, Amtrol, and Flexcon all have free sizing tools on their websites. Your local well contractor can also size a tank for you—many do it at no charge if you are buying the tank from them.

Account for the usable volume inside the tank

A pressure tank contains both water and air. The air is compressed as water enters, and when you open a faucet, the compressed air pushes the water out. Not all the water in the tank is usable—some stays behind because the air pressure cannot push it out completely. The usable volume is typically 40 to 50 percent of the tank's total size, depending on the tank design and your pressure settings.

Most sizing formulas and calculators already account for this and give you the total tank size you need to buy, not the usable volume. However, if you are doing the math by hand or using an older formula, ask your supplier whether the result is the total tank size or the usable volume. If it is usable volume, multiply by 2 to get the actual tank size to purchase.

Choose between bladder, diaphragm, and older steel tanks

Bladder tanks are the most common type sold today. They have a rubber bladder inside that separates the water from the air. The bladder is replaceable, so if it fails after 5 to 10 years, you can buy a new one instead of replacing the whole tank. Bladder tanks range from 2 gallons to over 100 gallons and work well for residential systems.

Diaphragm tanks use a rubber diaphragm instead of a bladder. They are less common but work the same way and are similarly priced. The diaphragm is also replaceable.

Older steel tanks without a bladder or diaphragm are still in use but are being phased out. Water and air mix inside, which causes the air to dissolve into the water over time. These tanks require periodic draining and re-pressurization to restore the air charge. If you have one and it is failing, replacing it with a bladder tank is usually the better choice.

Install the tank in the right location and check the air charge

Pressure tanks are usually installed in the basement, crawlspace, or utility room near the well pump and pressure switch. The tank should be on a level surface and supported by a stand or straps so it does not rest all its weight on the plumbing connections. Keep the tank away from direct sunlight and freezing temperatures, both of which can damage the bladder or diaphragm.

Before you put the tank into service, check the air charge—the pressure of the air inside the tank when it is empty. The air charge should be about 2 PSI below your pump's cut-in pressure. For a 30–50 PSI system, the air charge should be around 28 PSI. You can check it with a tire pressure gauge on the air valve (a small stem on top of the tank, similar to a car tire valve). If the air charge is too low or too high, use an air pump or let air out until it is correct. A wrong air charge reduces the usable volume and causes the pump to cycle more often than it should.

What to do if you are replacing an existing tank

If your current tank is failing and you are buying a replacement, check the size of the old tank before you order. The size is usually printed on a label or stamped into the metal. If your system has been running well with that size, you can buy the same size again. If the pump cycles too often (more than 10 times per hour) or if the water pressure drops when you use multiple fixtures at once, the tank is too small, and you should size up.

To count your pump's cycles, listen to it for a few minutes while someone uses water in the house. Each time you hear the pump start, count one cycle. If you hear it start more than 10 times in a minute, the tank is undersized. In that case, use the sizing formula above to find the right size for your current pump and pressure settings.

Frequently Asked Questions

Can I use a tank that is larger than the calculated size?

Yes, a larger tank will not harm your system and may reduce how often the pump cycles. However, a tank that is much larger than needed wastes money and takes up space. A tank larger than about 120 gallons also becomes difficult to fit in most basements or utility rooms.

What if my pump's flow rate is not on the nameplate?

Call the well driller who installed the pump—they keep records of the pump model and specifications. If the driller is no longer in business, you can identify the pump by its brand and model number (also on the nameplate) and search the manufacturer's website for the specs. Grundfos, Franklin Electric, and Goulds are common brands.

Does the tank size affect my water pressure?

Tank size does not change the maximum pressure your system delivers, but it does affect how steady the pressure stays. A larger tank holds more water, so pressure drops more slowly when you use water, and the pump does not have to start as often. A smaller tank causes pressure to drop faster, which may feel like weaker flow from your faucets.

How often should I check the air charge in my tank?

Check the air charge once a year, ideally in the fall before the heating season. The air charge can drift over time, especially if the bladder or diaphragm is beginning to fail. If you notice the pump cycling more often than usual, check the air charge first—it is often the culprit.

What happens if the air charge is wrong?

If the air charge is too low, the tank fills with water too quickly and the pump cycles more often. If the air charge is too high, the pump does not turn on until the pressure drops below the cut-in setting, which can cause low water pressure at the tap. Either way, the usable volume shrinks and the pump wears out faster.