How Does an Under Sink Water Filter Work?

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Under sink water filters are one of the most popular water filtration options among domestic users.

Many households have one nowadays, and purchasing a system for your home can be a great investment if you’re concerned about the quality of your drinking water.

If you’re curious about how exactly under sink water filters work, this article is for you.

Key Takeaways

An under sink water filter gets installed under a kitchen or bathroom sink. It filters the water before it comes out of the faucet above.

These are the different types of under sink water filters:

  • Single-stage system – Water runs through one filter element only.
  • Multi-stage system – Water runs through multiple filtration steps to remove more and different types of contaminants.
  • RO system – This uses a combination of filter stages, most importantly a reverse osmosis membrane with very tiny pores.

Also, some under sink water filters feature an inline design. They attach to the existing faucet. Others require a separate dispenser.

What Is an Under Sink Water Filter?

Under sink water filters are water filtration systems designed to seamlessly attach to your main plumbing and provide you with a constant supply of purified water.

Installed under the sink, they don’t take up any countertop space like filter pitchers do, and they provide you with clean water on demand, instead of having you wait until a certain amount has been purified.

How Does an Under Sink Water Filter Work?

The main idea behind an under sink water filter is that you hook it up to your plumbing and dispense water through your faucet as you normally would. Some under sink water filters also come with their own faucet, making it easier to install them in difficult spots.

Under sink water filters are mainly categorized based on their filtration process.

Single-Stage Under Sink Water Filters

Single-stage under sink water filters only run your water through one filter element, such as a carbon block. This usually results in less thorough filtration, but also comes with some benefits like taking up less space, working faster, and costing less.

Depending on the contamination levels you’re dealing with in your home, a single-stage under sink water filter could be perfectly sufficient for your situation.

Multi-Stage Systems

On the contrary, multi-stage systems run the water through several filtration steps, removing more and different types of contaminants and often also improving its quality in other regards.

For example, a multi-stage under sink system may incorporate a sediment filter in its first stage removing floating particles. This could be followed by a granular activated carbon filter ideal for removing chemicals and chlorine taste and odor. A carbon block filter in stage 3 could be used to get rid of even more chemicals and heavy metals.

Reverse Osmosis Systems

Under Sink Reverse Osmosis System

Reverse osmosis is a special type of water purification which uses a combination of high pressure and a membrane with tiny pores to remove contaminants. Water is forced through the membrane, which is designed to only let water molecules and other small elements through, leaving almost all contaminants behind.

Inline Design or Separate Faucet

An inline under sink water filter attaches directly to your plumbing and filters water before it reaches your existing sink faucet. This allows you to have your water filtered in an unobtrusive manner that doesn’t require you to use a separate dispenser or make any special changes to your kitchen’s layout.

On the other hand, some models have their own dedicated faucet for dispensing water. This means that you need to make room on your countertop for it, and you may also have to deal with a more complicated installation process that requires you to drill a hole in your countertop or sink.

How to Choose an Under Sink Water Filter?

  • Testing your water quality – testing your water quality before investing in an under sink water filter is important. It will allow you to identify the right model for your specific situation without wasting money on more advanced filtration methods which may be unnecessary. You can either test your water yourself using at-home kits, have it tested professionally at a lab, or request a copy of local water quality reports.
  • Finding the right filter technology – once you know what kinds of contaminants you’re dealing with, you must choose an under sink water filter that properly encompasses all of them. You should take some time to learn about different filtration methods and what kinds of contaminants they’re designed to remove.
  • Number of filter stages – more filtration stages mean more thorough filtering, but also make the system more expensive and usually bulkier. Many households can rely on a single-stage water filter just fine, while others may need to invest in a multi-stage or reverse osmosis system.
  • NSF/ANSI testing and certifications – there are specific NSF/ANSI standards for water filters. Some of them will indicate that the filter is designed to target specific types of contaminants, which may include chlorine, medications, heavy metals included lead, etc.
  • Water pressure and flow rate – larger households will usually require an under sink water filter capable of supporting higher pressure and flow rate levels. This is an important consideration if you live with multiple people, because you may suddenly find your sink dispensing less water than you would expect if you go with a lower-grade system.
  • System installation – contrary to what many people believe, under sink water filters are not that complicated to install. Many of these systems are designed to be easy to hook up by a layperson without any specialized experience. The most complicated part is usually cutting a hole through your countertop for the faucet, if your filter has its own one.
  • Filter life & maintenance cost – under sink water filters require relatively little maintenance. High-quality models can last for a long time with minimal additional work. The most you’d have to do is pay attention to when you need to swap out the filter cartridges. Other than that, as long as your filter system has been properly installed, it will require very little effort on your part to keep it running.
  • Space – one of the best advantages to under sink water filters is that they take up little space compared to other home filtration systems. Most people have plenty of free space under their sinks, and once the filtration system is installed, it will not obstruct your daily activities in any way. If you’re planning to purchase an under sink water filter with its own faucet, this can complicate things a bit, but it’s still a manageable situation for most households.
  • Price tag – cheaper under sink water filters usually come with various long-term problems, like requiring more frequent filter replacements or being prone to leaks. Spending a bit more on a high-end system is a worthwhile investment if you want some peace of mind, especially if you’re planning to install more than one filtration system in your home.

The Pros & Cons of Under Sink Water Filters


Unobtrusive Design

Under sink water filters are designed to be as unobtrusive as possible, operating in a spot where you can’t even see them most of the time. This makes them a convenient choice for smaller households with limited space available.

Low Maintenance

An under sink water filter typically requires less maintenance than most other filtration methods, and is ideal for busy households where minimizing ongoing chores is a plus.

plumber installing new under sink water filter in the kitchen

Easy to Use

You turn on your faucet and that’s it – you get clean, filtered water immediately. There are no additional steps involved, like filling a jug and waiting for water to pass through its filter.


Limited to a Single Faucet

One under sink water filter can typically only be used for a single exit point. If you want to cover more faucets, you’ll often have to invest in additional systems, and the cost of that can add up very fast in a large households with many points to cover.

Installation May Require Drilling

If you’re buying an under sink water filter with its own dedicated faucet, you’ll have to do some additional work during the installation, like drilling a hole through your countertop. This is something not everyone will be comfortable with, and may require the services of a specialist. That can add to the installation cost a bit, but usually the final price is still going to be manageable relative to the overall cost of the filter.

If you have any questions about how an under sink water filter works please don’t hesitate to leave a comment below!

About the Author Alexandra Uta

Alex is a content writer with an affinity for research and a methodical attention to detail. Since 2020, she has fully immersed herself into the home water treatment industry only to become an expert herself. Alex has been using water filters and similar products for years which has gained her lots of hands-on experience.
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