How to Select the Right Pressure Tank

Choosing the right pressure tank is about more than maintaining good water pressure. Your pressure tank also plays an important role in protecting your well pump and motor.

If a pressure tank is too small for the pump, the system may short cycle, causing the pump to start and stop too frequently. Excessive cycling can shorten the life of the pump, motor, pressure switch, and other system components.

It can also cause an installation to fall outside manufacturer requirements for minimum runtime or allowable cycling. Depending on the equipment and warranty terms, improper pressure tank sizing may affect or even void warranty coverage.

That's why pressure tanks should be sized according to the pump and system requirements, not simply by choosing a tank that looks large enough for the home.

The good news? Once you understand a few basic numbers, pressure tank sizing becomes much easier.

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What Does a Pressure Tank Actually Do?

In a conventional well system, your pressure tank stores water under pressure so the well pump doesn't have to turn on every time someone opens a faucet.

As water is used, pressure in the system gradually decreases. Once it reaches the pressure switch's cut-in pressure, the pump turns on.

The pump then supplies water and refills the tank until the system reaches its cut-out pressure, when the pump shuts off again.

For example, with a 40/60 PSI pressure switch:

40 PSI = Pump turns ON
60 PSI = Pump turns OFF

The water available from the pressure tank between those two points helps determine how long the pump remains off between cycles.

And that brings us to one of the most important concepts in pressure tank sizing:

Tank Capacity and Drawdown Are NOT the Same Thing

A common mistake is assuming that a 20-gallon pressure tank provides 20 gallons of usable water.

It doesn't.

A bladder or diaphragm pressure tank contains both water and compressed air. Because of this, only a portion of the tank's total volume is available as usable water between pump cycles.

That usable amount is called drawdown.

Tank capacity = the total internal volume of the tank.

Drawdown = the amount of usable water delivered between the pump's cut-out and cut-in pressures.

When sizing a conventional pressure tank, drawdown is one of the most important numbers to pay attention to.

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Why an Undersized Pressure Tank Is a Problem

This is where pressure tank sizing becomes especially important.

If the tank doesn't provide enough drawdown for the pump, system pressure can drop from the cut-out pressure to the cut-in pressure very quickly.

The pump turns back on.

Then it shuts off.

Then it turns back on again.

This repeated starting and stopping is called short cycling.

Short cycling can cause:

• Excessive pump and motor starts
• Increased wear on motor components
• Premature pressure-switch wear
• Additional stress on other system components
• Reduced pump and motor life
• Failure to meet manufacturer runtime or cycling requirements
• Potential warranty issues when an installation does not meet manufacturer specifications

⚠️ Bigger usually isn't the problem. Too small can be.

An undersized pressure tank can cause excessive pump cycling, shorten equipment life, and may affect manufacturer warranty coverage.

Always verify the pump, motor, and tank manufacturer's requirements when designing the system.

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So, What Size Pressure Tank Do You Need?

For a conventional well system, we need to know several things before selecting the tank:

1. Pump flow rate (GPM)
2. Required minimum pump runtime
3. Required drawdown
4. Pressure switch setting
5. Whether the system is conventional or variable-speed

Let's go through them.

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Step 1: Determine Your Pump's Flow Rate

Start with the approximate operating flow rate of your well pump, measured in gallons per minute (GPM).

For example, you may have a pump designed to deliver approximately:

5 GPM
10 GPM
15 GPM
20 GPM

But there's an important catch.

A pump's actual flow depends on the conditions it's operating against.

A pump labeled or marketed as a "10 GPM pump" will not necessarily deliver exactly 10 GPM in every installation.

Well depth, pumping water level, pressure requirements, pipe friction and other factors contribute to the system's Total Dynamic Head (TDH).

For new installations, the manufacturer's pump performance curve should be used to determine the pump's expected performance at the system's operating conditions.

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Step 2: Determine the Required Pump Runtime

The goal isn't simply to fit as much water as possible into the tank.

We want enough drawdown to allow the pump to operate for an appropriate amount of time when it starts.

Required minimum runtime can vary based on the pump, motor horsepower, motor manufacturer and system design.

Required pump runtime depends on the pump, motor, and manufacturer's requirements. For example, the Goulds Water Technology HydroPro sizing guide below identifies two minutes of pump running time as “recommended protection” and one minute as “adequate protection.”

Always follow the requirements and sizing recommendations for the specific equipment being installed.

HydroPro tank sizing guide. Always verify current manufacturer sizing requirements for the specific pump, motor, tank, and application.

Notice the difference between “recommended protection” and “adequate protection.” In this HydroPro sizing guide, Goulds recommends two minutes of pump running time for greater protection, while one minute is identified as adequate protection. This illustrates why pressure tank sizing should consider both pump flow rate and operating pressure, rather than simply choosing a tank based on its nominal gallon capacity.

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Step 3: Calculate the Required Drawdown

Once you know the pump's operating flow and required minimum runtime, you can estimate the required tank drawdown.

A simple calculation is:

Required Drawdown = Pump Flow Rate × Minimum Runtime

For example, suppose the pump produces:

10 GPM

and the system requires at least:

1 minute of runtime

Then:

10 GPM × 1 minute = 10 gallons of required drawdown

Here's the important part:

That does NOT mean you should buy a 10-gallon pressure tank.

You need a pressure tank capable of providing approximately 10 gallons of drawdown at your system's operating pressure.

The actual tank may have a much larger total capacity.

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Step 4: Know Your Pressure Switch Setting

Pressure settings affect how much drawdown a tank provides.

Common residential pressure-switch settings include:

20/40 PSI
30/50 PSI
40/60 PSI

The first number is the cut-in pressure.

The second number is the cut-out pressure.

So a 40/60 system turns the pump on around 40 PSI and shuts it off around 60 PSI.

Here's why that matters:

The same pressure tank can provide different amounts of drawdown at different pressure settings.

Manufacturers therefore publish drawdown charts showing how much usable water a particular tank provides at various pressure settings.

Once you know the drawdown your system requires, use the manufacturer's chart to find a tank that provides at least that amount of drawdown at your operating pressure.

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What About Pressure Tank Pre-Charge?

Bladder and diaphragm pressure tanks contain compressed air, known as the pre-charge.

The correct pre-charge depends on the system's operating pressure and the manufacturer's instructions.

For many conventional pressure-switch systems, the tank's pre-charge is typically adjusted to approximately 2 PSI below the pressure switch's cut-in pressure, with the tank completely empty of water.

For example:

30/50 PSI system → approximately 28 PSI pre-charge

40/60 PSI system → approximately 38 PSI pre-charge

The tank must be empty of water when checking or adjusting the pre-charge.

Always follow the pressure tank manufacturer's instructions for the specific tank being installed.

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Can You Install a Larger Pressure Tank?

In many conventional well systems, choosing a pressure tank with more drawdown than the minimum requirement is generally not a problem.

More available drawdown can allow the system to use more water before the pump needs to start again.

That can mean:

Fewer pump starts and longer intervals between cycles.

The practical tradeoffs are usually additional cost and the physical space required for the larger tank.

The bigger concern is going in the opposite direction:

Don't undersize the tank just to save space or money.

The pressure tank is part of the pump system—not simply a water-storage accessory.

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Pressure Tank Sizing Checklist

Before purchasing a pressure tank, make sure you know:

✓ Pump operating flow rate (GPM)
✓ Pump/motor minimum runtime requirements
✓ Required tank drawdown
✓ Pressure switch settings
✓ Tank drawdown at those pressure settings
✓ Conventional vs. variable-speed system
✓ Manufacturer installation and warranty requirements

If you're replacing an existing tank, don't assume the old tank was sized correctly simply because it was already installed.

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Need Help Selecting the Right Pressure Tank?

Still not sure which tank your system needs?

That's exactly the kind of question we're here to help with.

Give the Water Pumps Pro team your pump model, system pressure settings and application, and we can help determine the appropriate pressure tank for your system.