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Well Pressure Tank Sizing Guide: What Size Do You Need?

Scott Winfield
Last Updated on
by Scott Winfield

If you have well water, a well pressure tank is a water storage reservoir that can help keep your water pressure at a constant level.

If you’re putting in a new well or replacing an old well pressure tank, you might have some questions about which size well pressure tank you need. Several factors such as flow rate, pressure switch setting and minimum run time determine the pressure tank size you need and which size is best.

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What Size Pressure Tank Do I Need?

When you know the flow rate, minimum run time, and pressure switch setting, you can determine the drawdown. Knowing the drawdown will help you determine your tank size.

Pressure tanks come in 20-, 30-, 50-, 85-, and 120-gallon sizes. Size matters when it comes to well pressure tanks, so you’ll need to know a few things before you choose one:

  • Flow rate: What is your pump flow rate in gallons per minute (GPM)? The flow rate is usually initially tested when you drill a new well.
  • Minimum run time: How long will your pump run?.
  • Pressure switch setting: The pressure switch setting is the pressure at which the pump turns on and shuts off. The pressure switch setting can change the tank drawdown.
Well Tank Pressure Tank

There are three pressure switch settings for pressure tanks: 20/40, 30/50, and 40/60.

The first number is the lowest pressure (at which the tank turns on), and the last is number is the highest pressure (at which the tank turns off).

Most well pressure tanks come with a chart to tell you the drawdown capacity based on your pressure switch settings.

When sizing a pressure tank and determining the drawdown capacity you need, you can use these guidelines:

  • 0 to 10 GPM pump flow rate: 1.0 GPM of run time
  • 10 to 20 GPM pump flow rate: 1.5 GPM of run time
  • 20+ GMP pump flow rate: 2.0 GPM of run time

Once you know the pump flow rate and minimum run time, you can use the following equation to determine how much drawdown you need in your pressure tank:

Drawdown capacity = Pump flow rate x Minimum run time

Here’s an example of how these calculations might look:

15 GMP pump flow rate x 1.5 GPM minimum run time = 22.5-gallon drawdown capacity

In the above scenario, you would need to use a well-pressure tank with a 22.5-gallon or more drawdown capacity, which would be an 85-gallon pressure tank (more about that later).

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Can a Well Pressure Tank Be Too Big?

You can’t have a well pressure tank that’s too big. A larger tank would have longer and slower cycles, but the runtime would be the same.

In fact, bigger is better when it comes to well pressure tanks because they wear out more slowly with fewer startups and longer run times. Having fewer pump cycles increases tank longevity.

The only downsides to having a larger tank are that they cost more and take up more space. There’s also a chance it may pump the well faster than it could recover (although unlikely).

Thus, if space and cost aren’t an issue, it’s in your best interest to size up.

However, it is possible to have a tank that is too small. If your tank is too small, the pump will cycle too quickly, and you can wear your tank out faster, requiring a costly replacement.

The average person uses 101.5 gallons per day of water. The larger the number of people using your well water, the more work your well pressure tank has to do. 

A correctly-sized well pressure tank will last an average of 15 years. Some cheaper tanks only last five years, while high-quality pressure tanks can last up to 30 years.

A larger tank will last longer because it doesn’t have to work as hard. On the other hand, choosing a too-small tank will result in needing to replace it more quickly.

Is a 20-Gallon Pressure Tank Big Enough?

A 20-gallon pressure tank is only big enough if your pump flow rate is below seven gallons. It will only work below seven gallons because a 20-gallon tank only has six gallons of drawdown.

It’s important to understand that the volume of water that a well pressure tank can hold is not the same as the volume of its drawdown.

To determine if a 20-gallon pressure tank is big enough to both service your usage needs and protect pump longevity, you need to understand how much drawdown volume a tank has:

  • 20-gallon pressure tank: 6 gallons of drawdown
  • 30-gallon pressure tank: 9 gallons of drawdown
  • 50-gallon pressure tank: 14 gallons of drawdown
  • 85-gallon pressure tank: 25 gallons of drawdown
  • 120-gallon pressure tank: 36 gallons of drawdown

With the 6-gallon drawdown capacity of a 20-gallon pressure tank in mind, let’s look at the drawdown equation again:

6 GPM flow rate x 1 GPM minimum run time = 6 gallons of drawdown capacity

A pump that has a flow rate above 7 GPM will need more than 6 gallons of drawdown. Thus, a 20-gallon pressure tank is only useful if you have a flow rate at or below 6 gallons.

Even if you don’t need the capacity of a tank that’s bigger than 20 gallons, you may still want to invest in one.

Keep in mind that the larger the pressure tank, the better the tank’s overall life. So, even if your pump flow rate is only 0 to 6 GPM, a larger tank will serve you better for longer.

Will Installing a Larger Pressure Tank Increase Water Pressure?

Getting a larger well pressure tank will not increase your water pressure beyond the pump’s pressure control setting.

Thus, getting a larger pressure tank won’t improve your water pressure.

Pressure Tank

However, a larger pressure tank will cause the water pressure to decrease more slowly since it has a longer drawdown time. So, with a larger tank, water pressure will be higher for longer.

The best way to improve water pressure is to add a booster pump to improve water pressure and flow, including water pressure in buildings high above the waterline and on upper floors.

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What Is Well Pressure Tank Drawdown?

Well pressure tank drawdown is the amount of water that a pressure tank stores and makes available between the times when the pump turns on and off.

The tank’s drawdown capacity is the minimum amount of water the pressure tank can store and/or deliver between the time the pump shuts off and restarts.

Here’s an example of the action of a well pressure tank with a 30/50 pressure switch:

  1. The water pressure starts at 50 PSI.
  1. The water pressure gradually decreases down to 30 PSI.
  1. Once the pressure gets down to 30 PSI, the pump will activate again until it reaches back up to 50 PSI.

The volume of water that the tank contains when going from 50 PSI down to 30 PSI in this scenario is the pressure tank drawdown.

You shouldn’t confuse tank drawdown with tank volume. The tank volume is the tank size you need to get the drawdown you desire.

The bigger the well pressure tank, the more water you can store. If you have a larger drawdown, your tank can run longer and will go through fewer cycles.

How to Calculate Drawdown Capacity

You can use the following equation to determine how much drawdown you need:

Drawdown capacity = Pump flow rate x Minimum run time

Note: You should not use the above equation if your system has a variable frequency drive.

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Scott Winfield
Scott Winfield
My name is Scott Winfield and researching and writing about water filters and other strategies to purify water has become my full time passion in recent years. I'm glad that you found our site and you can look forward to authoritative and well researched content here to help you get the best in water.
  1. You point out how a larger tank will last longer but what about pump run time each cycle? If your pump has to run longer to fill the tank each time it comes on then will this wear your pump out sooner?

    1. Hi Richard, your point is logical. We believe that anytime a machine with moving parts does more work, it will hasten its lifetime or servicing required at its service intervals, generally speaking. That said, water pumps in these types of applications are engineered for exactly the qualities required such as endurance and longevity. In the final analysis, we believe that properly engineered systems that appropriately match the right pumps to the size of the tank bring into consideration all that is necessary to have a properly working and economical system. As always, if there are service intervals and preventative maintenance specified for in the system, all of those actions should be taken to ensure a cost effective and highly performing system.

  2. Would a larger tank give me the ability to run two sprinkler systems at same time ? Now if I turn on a 2nd prssure drops to much for them to be effective.

    1. Hi Gary, there are a lot of factors to consider about your specific system to say definitely. But, I’m confident that generally speaking, the situation you’re presenting is more about system design and making sure there is enough flow in terms of gallons per minute (GPM) and other factors related to how much water can flow versus how much water you have in reserve. So, it’s not about tank size as it is about the overall system’s ability to flow water. When you turn on a 2nd sprinkler system (or another load on your water system), you are stacking demands on the same VOLUME of water that your system is capable of delivering in any given period of time (per second/minute/hour). That’s why the pressure drops. So, it’s about right sizing your overall system’s capability to deliver water in a period of time rather than just the size of your tank. They are related as a larger tank can deliver a greater amount of water as needed by multiple sources. But, hopefully I’ve made the point clear. I would suggest a reputable water system business in your area to help ensure you have the right overall design for your needs.

  3. A larger tank is not always the answer. It may be the piping between the tank and the point of use. Smaller pipes cause more friction loss and may not be able to pass the amount of water being used. Case in point a 1/2″ pipe can not pass the same volume of water as a 3/4″ or 1″ pipe. The number of fittings also comes in to play as each fitting causes friction loss. Also it does little good to increase the size of the pipe at the point of use as you are limited to the volume and pressure of the water delivered to that point in the system.

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