Flow Rate and Sizing Guide

Flow Rate and Sizing Guide

What is My Home’s Water Flow Rate?


Water filtration equipment is typically sized in terms of flow rate – which is the volume of water passing through it at any given time.  This is typically expressed in gallons per minute (gpm).  

How To Calculate Flow Rate

When passing through water filters, softeners and media tanks, ultraviolet disinfection systems, or other water treatment devices, the water being treated requires a minimum amount of contact time with the filtration medium in order to remove the targeted contaminants.   Under-sizing equipment can result in insufficient contact time, and inadequate water treatment.  This is why when sizing equipment, it is important to know the flow rate you are treating, and is best to plan for peak flow rates.

There are several ways to estimate the flow in your home to allow for proper sizing of water treatment equipment, described below.

Supply Line Pipe Size

Check the pipe size at the entrance to your home or facility.   The size of the plumbing limits the flow of water, so this can be used as a guideline for the maximum flow rate coming into your home in extremely peak usage.

Estimated Flow Rates for Copper Piping
  Normal FlowPeak Flow
1/2" Pipe 4 gpm 7 gpm
3/4" Pipe 6 gpm 16 gpm
1" Pipe 16 gpm 30 gpm
1-1/4" Pipe 30 gpm 35 gpm
1-1/2" Pipe 40 gpm 70 gpm
2" Pipe 65 gpm 120 gpm 
2-1/2" Pipe 80 gpm 170 gpm
3" Pipe 120 gpm  270 gpm
Copper Piping Size Flow Rates

Estimate Usage by Appliances

Another way to determine your flow rate is to estimate the amount of water you will be using based on the fixtures and appliances in your home.  List out all of the fixtures and appliances that use water in your home, and look at the chart to get an estimate of how much water each will use.

Estimate the maximum amount of these that you would feasibly run at the same time – and add up the flow rate for each of these devices (or use our calculator below).  This gives you an estimated peak flow rate to use for sizing water filters, ultraviolet systems, or other water treatment equipment for your home or facility.   Note that your maximum possible flow rate will be restricted by the pipe size of your home or facility supply line. 

Estimate your water flow by appliances

Select the number of each fixture in your home and press 'calculate' to estimate your home's water flow rate.

Timed Fill for Individual Tap or Plumbing Fixture

The flow rate of an individual tap or plumbing fixture can be determined by measuring the amount of time it takes to fill a container with water.   For this method, you will need a measured container (such as a 1-gallon water bottle or 5 gallon bucket), and a stop watch.

Place the empty container beneath the faucet and open the faucet completely while starting the stop watch.  Stop the stop watch when the container is full and turn off the water.   The test may be repeated multiple times to ensure accurate results.   

Calculate the flow rate as follows:    60 ÷ [Seconds to Fill] × [Gallons Measured] =  GPM  
(or use our calculator below)

Enter how many seconds it took to fill the container, and how many gallons in the container used to measure then press 'calculate' to estimate your fixture's water flow rate.

Estimate your faucet flow rate by filling a gallon container

Now that I know my Flow Rate... 

How Do I Size My Water Treatment Equipment?

[Sizing Ultraviolet Systems] [Sizing Whole House Filters] [Sizing RO Systems]

Proper sizing of water treatment equipment is critical for effective water treatment. In some cases, such as ultraviolet water disinfection, under-sizing a system or not planning for peak flows can result in untreated water passing through the system and contaminating the water already in the pipes and equipment.  In other cases, such as whole-house-filter systems, choosing an undersized filter can restrict the flow of water coming into the facility, causing significant pressure and flow drop when more than one fixture or appliance is running.

Rule of Thumb for Sizing Water Treatment Equipment

Rule of Thumb for Sizing Water Treatment Equipment:  
When in doubt,  always choose the next size up.

Sizing Ultraviolet Water Disinfection Systems (UV Systems)

Question:  What Size UV System do I need?
Short Answer:  Choose a UV System with a recommended flow rate that meets or exceeds your peak flow rate.

Why is Appropriate Sizing Critical for UV Systems?

UV Water Treatment Flow RateThe ultraviolet (UV) treatment process is an extremely quick physical process. Ultraviolet light mutates and/or degrades DNA. DNA (or deoxyribonucleic acid) is the part of the cell that gives an organism its instructions on how to function and reproduce. When the DNA is damaged, the organism becomes unable to function because its “instructions” are garbled or missing. An organism that has no instructions cannot function and reproduce, and cannot cause infection. It is rendered harmless and eventually dies.

In the UV disinfection process, water is purified as it runs through a stainless steel chamber (also called a “reactor”) that contains a special UV- producing lamp. As the water flows past the lamp, the microbes in the water receive a lethal dose of UV. The water is then safe to drink. However, different organisms require different levels of UV energy to disrupt their DNA. This energy level is known as a UV dose.

What is UV Dose?

uv disinfection for waterUV Dose is expressed in mJ/cm2. The accepted standard for most applications is 30 mJ/cm2*. A slower flow rate will result in a higher UV dose. The longer the chamber and the longer the water stays in contact with the UV lamp, the higher the dose.   UV System manufacturers calculate the dose which can be expected at different flow rates in their system, and provide recommended flow rates that ensure a sufficient dose for disinfection. 

*For NSF 55 Class A requirements (which may apply where you live, check with your local municipal health region), UV dose is required to be 40 mJ/cm2.  

Exceeeding the manufacturer's recommended flow rate will shorten the time the water is in contact with the UV lamp, and lower the dose.  This may lead to insufficient disinfection, allowing organisms to pass through without deactivation.

Other Factors affecting UV Dose:

Different factors can affect UV dose and the ability of UV to deactivate organisms in the water. If there is turbidity (large amounts of particles) or water hardness, microorganisms can “hide” behind these particles. This hampers UVT, or UV transmittance, the amount of UV that actually penetrates the water. With a low UVT, “hidden” microorganisms will not receive the required dose of UV light to deactivate their DNA, and the water will not be properly disinfected.

Performing a water test will indicate what type of pre-treatment a UV system requires to increase UVT and produce maximum effective disinfection. The following chart shows the ideal state for maximum results:

UV Transmittance Measure of absorption of UV light, UVT can affect system sizing requirements >75% UVT @ 254nm
Iron Can affect both UVT and sleeve fouling <0.3 ppm (0.3 mg/L)
Hardness Can cause sleeve fouling <120 ppm (7 gpg)
Turbidity Can shield pathogen, post UV contamination <1 NTU
Tannins Can affect UVT and system performance <0.1 ppm (0.1 mg/L)

Sizing Whole-House Water Filter Cartridge Systems

Whole House Filter Cartridge Systems can restrict the water flow if the inappropriate size is chosen.  Choose the size that accomodates your estimated service flow.  

Flows up to 6 gpm: 10" Big Blue Cartridge Fitler Systems
Flows up to 10 gpm: 20" Big Blue Cartridge Fitler Systems
Flows greater than 10 gpm: Media Tanks - Contact us for a Quote
Whole House Water Filter

Sizing Reverse Osmosis Systems

Since Reverse Osmosis Water Treatment Systems are typically supplied with a storage tank, these should be sized based on estimated water usage volume rather than flow rate.   The water tank will hold the filtered water until you are ready to use it, and the system will continue to run only as needed to refill the tank and keep it with a ready supply of water.   The flow rate of an RO System operating on a tank will determine how quickly the tank will refill.



How to Size an RO System

Popular Whole-House Water Treatment Equipment

Below please find a selection of ultraviolet and whole-house water treatment equipment suitable for whole-home water treatment.   Please return to our home page to browse our entire product line.


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