How Do Under Sink Water Filters Work?

Author: Rory Mullan - Published: 2022/08/05 - Updated: 2022/12/22

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Under sink water filters are one of the most popular and convenient choices for water filtering systems available these days. Their popularity is thanks to their ease of use, affordability, and filtering capabilities.

But to know if these filters are right for you, you must first understand how an under sink water filter works. And that’s where this guide comes in.

We’ll walk you through the functionality of under sink water filters, including their different types, advantages, and disadvantages.

So, here is our guide on how under sink water filters work!


  • True to its name, an under sink water filter is placed under a kitchen or bathroom sink. Water is filtered before it exits the faucet above.
  • The many under sink water filter types are as follows:
    • Water only passes through one filter element in a single-stage system.
    • Water is filtered through a number of stages in a multi-stage system to get rid of more and various kinds of impurities.
    • A reverse osmosis system employs a number of filter stages, but its most crucial component is a semipermeable membrane.

What’s an Under Sink Water Filter and How Does it Work?

How Does an Under Sink Water Filter Work Thumbnail

An under sink water filter is a water filtration system integrated into your existing plumbing under a sink to supply you with clean and safe drinking water.

Unlike filter pitchers, they don’t take up any counter space and provide clean water on demand instead of having to wait until a certain amount has been purified.

Many under sink water filters are connected inline so that water can be dispensed normally through your faucet. Some under sink filters also come with their own dispenser, though.

There are several types of under sink water filters, and they are classified based on how they filter the water. These systems are:

Single-Stage Under Sink Water Filters

A single-stage under sink water filter runs your water through a single filter element, such as a carbon block. As a result, filtration is usually less thorough, but this has some benefits, like taking up less space, being quicker, and being cheaper.

If you have low levels of contaminants in your home, a single-stage under sink water filter may be perfectly adequate.

Multi-Stage Under Sink Water Filters

In multi-stage systems, water is filtered several times using different types of filtering media, removing a variety of contaminants while often also improving other aspects of its quality.

The first stage of a multi-stage under sink system is often a sediment filter that removes floating particles. In addition, a granular activated carbon filter[1] would be ideal for removing chlorine and chemicals from your water. In stage 3, a carbon block filter could be used to eliminate even more chemicals and heavy metals.

Depending on the type of multi-stage under sink water filter you are using, the number of stages and their order may differ.

RO Systems

In reverse osmosis, contaminants are removed using high-pressure membranes with tiny pores. In the process, water is forced through a membrane designed to only let through water molecules and few other small elements, leaving almost all contaminants behind.[2]

Inline Filter vs Extra Faucet

Inline under sink water filters attach directly to the plumbing and filter the water before it reaches the sink faucet. Using this method, you can filter your water without having to use a separate dispenser or make any special changes to your kitchen’s design.

However, some models feature their own dedicated water faucet, which means you will need to make room for it on your countertop, and you may also need to drill a hole into your countertop or sink in order to install it.

under sink water filter being installed

Things to Consider When Choosing an Under Sink Water Filter

Consider the following factors before choosing an under sink water filter system for your home:

  1. The quality of your water – Before investing in an under sink water filter, you should test your water quality. This way, you can identify the right model for your specific situation without wasting money on more advanced filtration methods that may be unnecessary. At-home water testing kits, lab tests, and consumer confidence reports are all ways to analyze your water.
  2. NSF/ANSI certifications – There are specific NSF/ANSI standards for water filters. Filter manufacturers may indicate that a filter is designed to remove certain kinds of contaminants, such as chlorine, medication, heavy metals like lead, etc. NSF/ANSI certifications are a way of verifying that independent labs have tested a manufacturer’s claims.
  3. The type of filter system – Choosing a filtration system with more stages implies more thorough filtering, but it is also more expensive and generally bulkier. While some households can do just fine with a single-stage water filter, others may need to invest in a multi-stage or reverse osmosis system.
  4. The filtering media – You must choose an under sink water filter that appropriately encompasses the types of contaminants you’re dealing with once you know about them. Consider learning about the different types of filtration methods available and what contaminants they are designed to remove.
  5. The size of your home – A large household will typically require an under sink water filter that can handle higher pressure and flow rates. Otherwise, if you live with multiple people, a slow under sink filter might lead to long waiting times.
  6. The filter’s size – In comparison with other home filtration systems, under sink water filters take up very little space. A filtration system can be easily installed under most sinks, and once installed, it will not hinder your everyday activities. An under-the-sink water filter with its own faucet can complicate things a bit, but for most households, it’s still manageable.
  7. The installation process – Under sink water filters are much easier to install than many people think. The majority of these systems are easy to hook up by anyone without specialized training. Generally, the most difficult part is cutting a hole through your countertop for the faucet if your filter has one.
  8. The price tag – A lower price tag usually means longer-term problems, such as requiring more frequent filter replacements and leaks. For peace of mind, investing in a high-end system is worthwhile, especially if you’re planning to install multiple filtration systems in your house.
  9. Maintenance costs – Under sink water filters usually do not require a great deal of maintenance. When built properly, high-quality models can last for a long time with only minimal work required. All you have to do is watch when the filter cartridges need to be changed.

Advantages and Disadvantages of Under Sink Filters

Like with any filtering system, under sink filters have upsides and downsides. For instance, while they take up little space and require almost no maintenance, they are limited to one faucet. Here is the full list of pros and cons:


The advantages of using an under sink water filter include:

Ease of Use

With under sink water filters, it’s just a matter of turning your faucet on to get clean, filtered water right away. You don’t have to add any extra steps, such as filling a jug and waiting until the water passes through the filter.

Easy to Maintain

Typically, under sink water filters require less maintenance than other filtration methods, and they are ideal for busy households that prefer to minimize ongoing chores.

under sink reverse osmosis system with storage tank

Handy Design

Most under sink water filters are designed to be as unobtrusive as possible, running in a spot you can’t see most of the time. As a result, they’re the perfect choice for smaller households that have limited space.


The downsides to under sink filters are:

Single Point of Use

Typically, an under sink water filter can only be used at one exit point. In a large household with many faucets to cover, you’ll often have to invest in additional systems, and that can add up very quickly.

You May Have to Drill into Your Countertop/Sink

You might have to do some additional work during installation if you choose an under sink water filter with a dedicated faucet, such as drilling a hole through your countertop.

Some people may be uncomfortable with this, and a specialist may be required. The installation cost is likely to be a little higher because of that, but the final price is usually still manageable compared to the overall price of the filter.


Under sink water filters are great choices because they don’t require a lot of maintenance or space. With single-stage filters, you get a simple system that will provide sufficient filtering capabilities for most households. And if you need something more powerful, you can always opt for a multi-stage system.

One downside to under sink filter systems is that while they are generally easy to install, they do require additional work in cases where an extra faucet is needed. Nevertheless, they are still one of the most practical options out there.

Further Reading


Meet Rory Mullan

Rory Mullan Rory has joined the Water Masterz team as a contributing writer. He has covered all sorts of topics in the last several years.

Outside of his writing work, Rory enjoys photographing the Irish landscape and making music!

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