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How Do You Size a Water Filter?

Correctly sizing a water filtration system is a little more difficult than you might’ve thought initially. However, once you take note of all the important steps in this article, you’ll have a far easier time deciding which one is right for you.


Feel free to take notes from this article for future reference in case you plan on moving to a new home. And don’t forget to share it with other friends that might find the following information useful.


The Basics of Sizing a Water Filtration System


Often times, you’ll find that water filters come with indicative sizing references such as “1 – 3 bathrooms”. Be sure to follow these indicators because they will give you an estimate on which you can rely.


Another important factor that plays a role in choosing the correct size of a water filtration system is the number of people in your household. Let’s say you only have two bathrooms, but there are about seven people who constantly use them. That complicates things a little because the average consumption of water inevitably increases.


Moreover, the more you use appliances that require water, such as dishwashers and washing machines, your consumption will also increase, and the filter’s capacity also needs to be bigger.


Sizing Units of Measurement


When looking for a reverse osmosis system type of filter, you’ll often find that the measurement is taken in GPD (gallons per day), and when looking for a whole house filtration unit, the measurement is taken in GPM (gallons per minute).


In short, the water needs to flow at a certain level for the filtration media to be able to capture all the possible pollutants. If your water filtration system isn’t big enough for the volume of water you use, your system won’t be able to effectively clean your water.


To help you out in measuring how many gallons your household regularly consumes, we will present a list of how much water the major appliances and fixtures you might have take up. Get your calculator ready and sum up how many total gallons you’ll be using in a single minute or in a single day.


We also encourage you to sift through these products’ user manuals because you will usually find information regarding their water intake there.


faucet water filtration system


Estimates of Water Consumption per Appliance


  • Bathtub: 4 to 8 GPM
  • Washing machine: 4 to 5 GPM
  • Shower: 2.5 to 5 GPM
  • Toilet: 2.2 to 5 GPM
  • Dishwasher: 2 to 3 GPM
  • Faucet: 2.5 to 3 GPM


Again, we urge you to look over the technical specifications of each of these products that you have. These estimates are only to help you out in figuring out the approximate size of the filter you’ll need.


Check the Pipe Size


Another way to determine what size water filtration system you need, that requires less math, is to check the size of your line pipe, which is the pipe at the entrance to your home.


Because the size of the line pipe determines the volume of water, it can be a good tool to estimate what size filtration system you would need. For example, a 1” line pipe typically has a flow rate of minimum 16 to a peak of 30 GPM.


Chart Regarding GPM and Water 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.


Pipe size

Minimum GPM

Maximum GPM

½-inch pipe



¾-inch pipe


16 GPM

1-inch pipe

16 GPM

30 GPM

1.25-inch pipe

30 GPM

35 GPM

1.5-inch pipe

40 GPM

70 GPM

2-inch pipe

65 GPM

120 GPM

2.5-inch pipe

80 GPM

170 GPM

3-inch pipe

120 GPM

270 GPM


Calculating the Time to Fill For Individual Taps and Fixtures


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.


To do so, you will need a container, preferably a one-gallon water bottle, or a five-gallon bucket. And, obviously, a method to measure the time it takes to fill the container, such as a stopwatch or a stopwatch app on your smartphone.


Place the empty container beneath the faucet. Open the faucet completely while starting the stopwatch. When the container is full of water, stop the watch and see the results. You can repeat the test multiple times to get a more accurate reading.


Use the following formula to calculate the average GPM:


60 ÷ seconds to fill x measured gallons = GPM


How to Size a UV Water Filtration System


The same principles apply even when choosing a UV water filter. However, the UV filter’s capacity to kill viruses, bacteria, and other pathogens depends on the exposure time of the water to the UV-C rays.


Traditional UV water filters use a quartz tube that houses a UV lamp which kills bacteria by disabling its DNA. The problem is that the tube and lamp are straight and don’t expose the water to the intended amount of UV-C rays for long periods.


This is where the Yarna UV Filter comes in. It’s made with SpiralLight technology that uses a - you guessed it - spiral water flow tube to get the water exposed to the UV-C rays for longer. We have two models available: 13 GPM or 18 GPM.


If it sounds too good to be true, we’re offering you a complete one-year money-back guarantee in case you’re not satisfied with our product. And if you choose to register it on our website, we’ll also give you an extended 10-year warranty.


The Bottom Line


Sizing a water filtration system may be a tedious process, but it’s not impossible by far once you learn all the techniques presented in this article.


In fact, it becomes quite easy once you start applying some basic math and taking into consideration how much water you consume on a daily basis. With all that said, we wish you good luck in sizing your next water filtration system!