How to Size a Well Pressure Tank

Posted by
John Woodard on March 27, 2024

A pressure tank is an integral part of maintaining your well. Pressurized well tanks extend the lifespan of your well pump by preventing rapid on/off cycling of the pump and maintaining water pressure throughout your home. A properly sized pressure tank ensures your household water needs are met and protects your pump from short cycling. In this article, you can learn what well pressure tanks are, how to size them, and what accessories you need to make sure the tank functions properly. 

What is a well pressure tank? 

well pressure tank maintains the water pressure in the household and protects the life of the well pump. Every time a faucet turns on, a toilet flushes, or someone runs a bath, water is demanded from your well. The pressure tank holds a quantity of water to meet these needs before asking the well pump to kick on. This extends the time between the pump turning on and turning off. If that on/off cycle is fast, every time you open a faucet the pump will turn on, and when you close the faucet, the pump shuts off. When the well pump is turning on and off too frequently, it is known as “short cycling”. Short cycling puts enormous stress on your well pump and can cause it to prematurely fail. This requires an expensive replacement that could have been avoided. 

By extending the time between the on and off cycles, a pressure tank keeps your well pump from undue wear and tear. It also stabilizes your overall household water pressure, so the flow rate coming out of your faucets remains constant.

How does a well pressure tank work? 

A well pressure tank uses compressed air to push pressurized water out of the tank and into your home. Pressure tanks have a diaphragm, also called a bladder, that separates a chamber of air from the water. As the tank fills with water, it compresses the air chamber. When you turn your water on, the compressed air pressurizes the water and pushes it back out of the tank. 

How does a pressure switch work on a well pump?

Pressure tanks are set up to run in conjunction with a pressure switch. The pressure switch monitors the pressure in the tank. It informs the well pump when to turn on and off based on the rise and fall of the pressure within the tank. Most households are set up to run with a pressure switch that turns the pump on at 30 psi and switch the pump off at 50 psi. The gauge in front of the tank indicates when the maximum pressure has been reached, signaling the switch to automatically shut off the pump.

Learn more: What is a well pressure switch?

What is drawdown?  

Drawdown is the amount of usable water in the pressure tank. It is the volume of water drawn from the tank between the time the pump takes to turn on and off. As water comes out of the tank, the pressure of the tank decreases. If you are using a 30/50 pressure switch, the pressure will continue to drop down until it reaches 30 psi. At this point, the pressure switch will activate the well pump until the tank reaches 50 psi. That volume of water between 50 psi and 30 psi is the drawdown. Essentially, the drawdown is the length of time the pump is protected from shutting on and off. Drawdown is not to be confused with the total tank volume. Your total tank volume is a measure of the tank size required to produce the desired drawdown capacity. 

What size well pressure tank do I need? 

Sizing a well pressure tank is a three-step process of knowing the flow rate, the minimum runtime of the pump, and the pressure switch setting. These three variables must be calculated together to find what size pressure tank works for a system. Follow these steps to properly size your well pressure tank: 

1. Flow Rate

The flow rate of a pump is how many gallons per minute it produces. A single-family home can get by with a 6 gpm pump in many cases, but some homes may need more depending on water usage, number of bathrooms, and number of occupants in the house.

2. Minimum Runtime 

Multiplying the flow rate by the minimum runtime of the pump will give you the drawdown capacity. As a rule of thumb, any pump operating at 10 gallons per minute (GPM) or under should be producing one gallon per minute of runtime.

10 GPM flow rate x 1 = 10 gallon drawdown capacity

Anything above 10 GPM should be producing 1.5 gallons per minute of runtime.

16 GPM flow rate x 1.5 = 24 gallon drawdown capacity

If your flow rate is above 20, you may require multiple tanks.

3. Pressure Switch Setting

The pressure switch setting is the pressure at which the pump turns on and off. The three pressure switch settings for pressure tanks are 20/40, 30/50, and 40/60. The first number is known as the cut-on pressure, the pressure that will activate the pump as the tank becomes too empty. The second number is known as the cut-off pressure, a limit that is reached when the tank fills enough to reach this pressure threshold. The cut-off pressure has the most direct effect on your drawdown capacity. For example, if you have a 40/60 switch, your drawdown capacity is less than a system with a 30/50 switch. Most well pressure tank manufacturers will provide you will a chart to tell you what your drawdown capacity is based upon the pressure switch setting.

What is a tank package?

When purchasing a new pressure tank, it is important to also purchase a corresponding tank package, also called a t-pack. The t-pack attaches to the outlet of the well tank and hooks up to your inlet line from the wall. These come with many of the necessary parts to operate and monitor your well pressure tank, including the pressure switch, pressure gauge, relief valve, tank cross, and ball valve. 

Learn more: What is a water storage tank and how does it work?


If you have any additional questions, please do not hesitate to contact us.
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