How to Determine Your Well Pump Flow Rate on Wells with Pressure Tanks
The first step is to count the gallons “drawn down” from the pressure tank. This is the amount of water the pressure tank stores before your well pump needs to kick in. This is known as pressure tank ‘drawdown'.
Then the next step is to time (in seconds) how long it takes for your well pump to turn on and then turn off. This is referred to as the cut-in and cut-off cycle of the well pump.
You just need those two numbers: The amount of water in gallons, of the drawdown of the pressure tank; and the time in seconds it takes for the well pump to fill up an empty pressure tank!
Here are the Easy Steps To Take
First, allow the well pump to build up to full pressure by running water in the home or from a hose bib or faucet until you hear the well pump kick in or pressure switch points click.
Close faucet and make sure no water is being used in the home. Well pump will build up pressure for a few minutes and then pressure switch will shut off well pump.
Next, open a hose bib (also referred to as a ‘spigot’ or ‘faucet’) anywhere near or after the pressure tank.
Run the water into a 5-gallon bucket, noting how many gallons you collect. If bucket fills up, dump out water and keep measuring and filling.
Measure the number of gallons drawn down from the pressure tank until the well pump turns on.
When you hear the pump turn on, immediately close the hose bib.
Time the period in seconds it takes for the well pump to build back up to pressure that is, between the cut- in and cut- out of the pressure switch. The pressure switch turns on the pump at a lower pressure and turns off the pump when it reaches a higher pressure.
The formula for determining the flow rate is gallons drawn down (that were measured above), divided by the seconds required for recovery, then multiplied by 60:
(Gallons / Seconds) x 60 = Gallons per Minute (GPM) flow rate..
For example, if 20 gallons are drawn down and it takes 120 seconds to build pressure back up, then: 20 divided by 120 = .166. Then multiply .166 x 60 = 10 gallons per minute flow rate.
So you have 10 GPM flow rate.
Introduction
Knowing your well pump’s flow rate is key to keeping your water system efficient and reliable. Flow rate, measured in gallons per minute (GPM), affects your daily water supply and the performance of filters like iron or acid neutralizers. Your pressure tank plays a central role by regulating water pressure and storing water for immediate use. By measuring your flow rate and the pressure tank’s drawdown, you can check if your system is sized correctly. This helps prevent low pressure, short cycling, or poor water supply, and ensures your system delivers the GPM you need.
Understanding Pressure Tanks
A pressure tank helps maintain steady water pressure and reduces how often your pump turns on. It stores pressurized water so the pump doesn’t run every time you use water. This reduces wear and saves energy. Pressure tanks come in different sizes, and getting the right one is important for performance and durability. Pressure switch settings (like 30/50 or 40/60 psi) control the range of pressure in your system and affect how much water is available before the pump restarts. A correctly sized tank and proper pressure settings help your system run smoothly and protect your pump.
Factors Affecting Water Flow
Many things can affect water flow in your well system. Pump size and type determine the flow rate. Variable speed pumps adjust to your water use and maintain steady pressure. Single-speed pumps may struggle during high demand. Tank size and pressure switch settings affect drawdown and how long your pump runs. Pipe size, flow restrictors, and your main supply line also impact water flow. Understanding these factors helps ensure strong, steady water pressure for everyday use.
Determining Drawdown Capacity
Drawdown capacity is the amount of water your tank can deliver before the pump turns on. To calculate it, multiply your pump’s flow rate (GPM) by the desired pump run time (in minutes). For example, a 10 GPM pump with a 1-minute run time needs a tank with 10 gallons of drawdown. This formula helps you pick the right tank size, reduce short cycling, and maintain good water pressure.
Measuring Flow Rate with Pressure Settings
To measure flow rate using your pressure tank, start with all water off in the home. Open a faucet or hose bib to drain water until the pump turns on. Once it does, close the faucet and time how long it takes the pump to shut off. That’s your run time. Use your tank’s drawdown (the water delivered during this cycle) and divide it by the time in minutes to get your flow rate:
GPM = Drawdown ÷ Time
For accuracy, you can use a flow meter or a 5-gallon bucket and a stopwatch. This method helps you confirm your system’s performance and plan for maintenance or upgrades. Measuring flow rate with your pressure tank is a simple way to ensure your system meets your water needs.
Resources:
- How Do Well and Pressure Systems Work?
- Learn How to Troubleshoot Low Water Pressure on Well Water Systems
