Flow Rate: How to Size a Whole House Water Filter

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When you need to buy a whole house water filter, its filtration properties are not the only factor you need to watch out for. You must also be careful about choosing the right system size for your current situation.

Purchasing a whole house water filter that’s too small can cause issues with your water flow, while an oversized one will lead to unnecessary expenses, both in the initial purchase and in the long term. – and that’s not all!

So how to size a whole house water filter the right way? Find out below.

Key Takeaways

  • Your ideal size of whole house water filter will depend on your required water flow rate.
  • Flow rates can be estimated based on the size of your household: # of bathrooms, # of people, combined flow rate of appliances (preferred), or square footage.

Why Size Matters

A whole house water filter works by running water through one or more stages of filtration designed to remove contaminants.

Since the filter acts as a bottleneck in your plumbing system, choosing one that’s too small will restrict flow which you will notice at times of peak water usage around your home. Pressure from your shower head goes down, flow from the kitchen faucet becomes a sad trickle, etc.

On the other hand, a whole house water filter that’s too large won’t cause any problems with your water flow or pressure, but it will be an unnecessary expense. Not just upfront, when you’re buying the system itself, but also in the long term, as replacement filters for larger models typically cost more than smaller-sized ones. Larger filters also tend to waste more water in case they utilize backwashing.

Kitchen Faucet

Guide: How to Size a Whole House Water Filter

Understanding the importance of choosing the right whole house filter size is key. Many people get confused at this stage in the buying process as it involves juggling a number of different variables. Don’t worry though – things aren’t as complicated as they might seem!

So, what size whole house water filter do you need?

How to Calculate Your Water Flow Rate

Several methods can be used to determine your required water flow rate. They will usually give different results, making it important to take as many different measurements as possible if you want to buy the ideal filter model.

# of Bathrooms vs # of People

As a rough estimate, you can simply take a look at the number of bathrooms or number of residents in your household. The latter is especially useful for determining the ideal size of a whole house water filter. The number of bathrooms is also a good indicator, but it might not tell the full story in some homes.

Let’s look at an example: A household with 1-2 residents and 1-2 bathrooms requires a flow rate of around 5 to 7 gpm (gallons per minute), while doubling the number of residents brings that up to about 10 gpm.

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#of People 1-2 Baths 2-3 Baths 3-4 Baths 4-5 Baths 5-6 Baths
1-2 5-7 gpm 7 gpm 7-10 gpm 10-12 gpm 12 gpm
2-3 5-7 gpm 7-10 gpm 10 gpm 12 gpm 12-14 gpm
3-4 7-10 gpm 7-10 gpm 10-12 gpm 12-14 gpm 14 gpm
4-5 7-10 gpm 10-12 gpm 12 gpm 14 gpm 16 gpm
5-6 10 gpm 12 gpm 12-14 gpm 16 gpm 18 gpm
6-7 10-12 gpm 12-14 gpm 14 gpm 16-18 gpm 20 gpm
7-8 12 gpm 14 gpm 16 gpm 18 gpm 20 gpm

Calculating Flow Rate by Appliances

You can also check how much water comes out of different access points, including appliances like washing machines. Most appliances should have their water flow rate listed in their specifications – if not, you can use 3 gpm as a rough estimate for most washing machines and dishwashers.

To measure the flow rate at a specific access point, like a faucet, place a bucket of a known volume underneath and fill it with water while timing with a stopwatch. Then just divide the volume by the number of seconds, and multiply by 60. For example, if it takes 100 seconds to fill a 5-gallon bucket, that faucet’s flow rate is about 5 / 100 * 60 = 3 gpm.

Appliance Water Use
Kitchen Sink Up to 3 gpm
Bathroom Sink 1-2 gpm
Shower 2.0-2.5 gpm
Tub Up to 5 gpm
Toilet (Low-Flush) 1.6 gpl
Toilet (Standard) 5 gpl
Outside Hose (½-Inch) 5 gpm
Dishwasher 6.0-16.0 gpl
Washing Machine 25.0 gpl

Calculating Flow Rate by Square Footage

The square footage of your home could also be used as a rough guide for estimating its flow rate, though this is far from precise. There are many additional variables that come into play here, so using this type of estimate should be left as a last resort.

By the way, the standard size of a new single-family US home in 2021 was 2,356 square feet.

What Is a Typical Home Water Flow Rate?

A standard 2-person household may vary from 5 gpm to 12 gpm or even more, depending on the square footage and number of bathrooms. Note that the flow rate at access points will always be limited by the points themselves, as some faucets can only let so much water through.

Filter Port Size

Check the size of the whole house water filter’s port too. You should always use a filter with a port size of at least 1”, even if your piping is smaller than that. Otherwise, you risk unnecessarily restricting your water flow.

Check Your Pipe Size

If you have easy access to your main plumbing, you should take a look at the size of your pipes, too. This can also give you a good idea of what kind of flow rate you’re dealing with, or at least its upper bounds.

Water Flow Rate vs Water Pressure

Some people use flow rate and water pressure interchangeably. While the two are related, they are two different variables. Flow rate is directly affected by water pressure – the higher the pressure, the stronger the flow. However, there are still limitations to the flow rate you can achieve with pipes of a certain size.

How Does Flow Rate Affect Water Filtration?

Higher flow rate usually leads to faster filtration, provided your whole house filter is capable of handling it. In the end, the filter acts as a gate through which your water passes, so increasing the flow rate means that more water will end up running through that filter. As long as you don’t hit the upper limits of the filter’s own flow rate, this will always lead to an increase in the speed of your filtration.

Why Is Water Pressure Important?

Having the right pressure in your plumbing is more important than it seems, and it determines more than simply how fast water comes out. Incorrect water pressure can lead to problems with your plumbing, including permanent damage to piping or fixtures. This applies to low pressure as well as for high.

Common Whole House Water Filter Sizes & Flow Rates

4.5” by 20” is probably the most commonly found whole house water filter size on the current market. Here, most filters start at around 10 gpm, which should be sufficient for most 1-3 person households. However, as the size of your house grows, you might need to consider a larger filter with a better flow rate to make up for the additional consumption.

If you have any questions about how to size a whole house water filter please don’t hesitate to leave a comment below!

About the Author Gene Fitzgerald

Gene Fitzgerald is one of the founders of BOS and currently head of content creation. She has 8+ years of experience as a water treatment specialist under her belt making her our senior scientist. Outside of BOS, Gene loves reading books on philosophy & social issues, making music, and hiking.
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