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Home » Water Filtration Systems » How to Install an Under Sink Water Filter (& How Much it Costs)

How to Install an Under Sink Water Filter (& How Much it Costs)

By: Craig Smith
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How to Install an Under Sink Water Filter and How Much it Costs

Are you tired of raiding your savings just to pay for all the bottled water you’re buying? Does your tap water taste so bad that you’d volunteer to have your fingernails yanked out rather than drink it?

The good news is that an under-sink water filter can take care of your water quality issues. By installing one of these, you can avoid taking out a mortgage to pay for bottled water and you won’t have to submit yourself to torture.

In this post, we describe the installation process for an under-sink water filter system. We explain the steps to installing the filter yourself, and we discuss what it would cost to have a professional plumber take care of installation of the filtration system for you.

Under-Sink Water Filter Installation

While you can hire a professional to install your under-sink water filter, many filters are designed for easy installation by the homeowner. Just follow our steps below and you can save hundreds over hiring a plumber.

Required Tools and Supplies

Most under-sink water filters come with detailed installation instructions. Typically, you need some common tools, such as:

  • Under-sink water filtration system
  • A utility knife or other way to cut plastic tubing
  • Adjustable wrench
  • Screwdrivers
  • Towel or small tray to catch drips
  • Teflon tape
  • Drill 
  • Hacksaw (for reverse osmosis systems)

Shut off the Cold Water Supply Valve

An under-sink water filter usually attaches to the cold water supply. Locate the cold water supply valve and turn it off. Make sure to turn your kitchen faucet on afterwards to verify you shut the correct line off. 

Remove the Cold Water Line

You need to disconnect the line from the shutoff valve to the faucet. Unscrew it from the shutoff valve. There should be a small amount of water drip from the water line, so place a towel or small tray underneath it to catch the small amount of water you can expect.

Install the Adapter

You’ll need to install an adapter to the cold water line. This is typically a t-adapter fitting, and many water filters include it when you buy the filter. If yours doesn’t include one, you’ll need to provide one to fit on the water valve where you disconnected the line to the faucet. 

Make sure to wrap the threads with teflon tape, then thread the fitting into the valve and tighten it.

Once you install that fitting, connect the water line from your faucet to one outlet on the valve.

Drill a Hole for a New Faucet (Depending on Design)

Your kitchen sink may already have a location for a dedicated faucet for a drinking water filter.

Many sinks have a knockout to open the hole up. You may already have a soap dispenser occupying that location, in which case you’ll still need to drill another hole. The typical size you need is 1 ⅜ inch, but check your instructions to be sure.

Depending on your kitchen sink design, it may not have this knockout. If that’s the case, you must drill a hole to mount the dedicated faucet for your filtered water. Be careful when deciding where to drill the hole. Make sure it is close enough to where you plan to mount the filter so the supply line can reach.

Some installers decide to put the new faucet on the countertop, not on the sink itself. This may be the best solution for some sink designs, or if you already have a soap dispenser and don’t have room for your filter tap. 

Install your New Faucet

You might need a second person for this task. 

Position the new faucet into the correct location, poking down through the hole you prepared above. Some faucets come with a rubber gasket that fits between it and the counter or sink. In some cases you may need to add a silicone sealer instead when you position the faucet.

Have your helper hold the faucet in place while you tighten the nut on the bottom to secure in place. It needs to be snug, but don’t overtighten it.

Install Branch Tailpiece (For a Reverse-Osmosis System)

If you have a revers-osmosis filter, you’ll need to connect it to a drain. Your filter should have included a branch tailpiece, which is an adapter with a connector on the side for the RO system to connect to. If you need one, these are available in any hardware store. You can also buy one like this one from Plumb Pak for under $10.


You can often remove the tailpiece between your sink drain and the trap and replace it with the branch tailpiece. Use the old tailpiece as a guide to mark the length of the branch tailpiece, then cut the branch tailpiece using a hacksaw.

Mount the Filter

If you haven’t done so already, figure out the precise location you want to locate your under-sink filter system. Make sure it is close enough to the new faucet for the water line to reach easily and be sure there is no interference from existing lines or pipes in your under sink space.

You’ll need to predrill for the mounting screws. Hold the filter or the mounting bracket in place and mark where the screws should be. Use a drill to predrill the mounting holes. You can then screw the filter in place.

Connect the Filter to the Plumbing

There should be a water inlet on your under-sink water filter. Connect a line from the fitting on the cold water valve to the filter. 

There should also be an outlet opening on the filter. Run a water line from this location to your new faucet. 

If you have an RO system, run the drain line from the filter to the drain.

Turn the Water Back On

You’re now ready to turn the water pressure back on. Turn the water supply valve on slowly to avoid sudden pressure changes that could damage your filter. Press the pressure release on your water filter to release trapped air.

Inspect for Leaks

Look carefully under your sink to make sure there are no leaks. Pay special attention to the cold water valve, t-adapter, and the new water lines you ran. If you find any leaks, correct them immediately. In some cases, snugging fittings a little may be all that is necessary.

Flush the System

Run water from your new faucet for about five minutes to flush out your new filter and cartridges. You’ll also need to do this after installing replacement filters.

Cost for a Plumber to Install an Under-Sink Water Filter 

Under-sink Filter Cost

You’ll pay $50-500 for most under-sink water filter systems. This varies based on the size, number of filter stages, and type of system you choose.

Professional Installation

While installing under-sink water filters is not complex, you may feel more comfortable having the installation done by a plumber.

There are several factors that go into what you’ll pay for installation. These aren’t limited to, but include:

  • Where you live
  • The size and complexity of your under-sink water filtration system
  • How the faucet mounts and the type of sink or counter

On average, you can expect a plumber to take anywhere from one to three hours to install a water filter under your sink. You’ll pay $125-320 on average. 

Other Costs

You may require additional materials, such as:

  • Valves
  • Adapters
  • Water lines
  • Drain lines

Your installation my also cost more if you have a reverse-osmosis filter. This is because reverse-osmosis systems require installing a drain line. That could add to the labor costs and could require additional materials.

In Conclusion

Many people can handle under-sink water filter installation. If you follow the installation process above, read the instructions that came with your new filter, then you can probably install it yourself. 

This can save a lot of money over hiring a plumber. If you decide to let a plumber handle installation of your new under-sink filter system for you, you can expect to pay about $125-320.

 Either way, with your new under sink water filter system in place you’ll enjoy pure drinking water.

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Craig Smith
Craig got his start in water working in the swimming pool and spa industry. Water treatment would grow into his main career but he ended up working in the pool industry for over 26 years where much of his time was spent balancing the water in customer's swimming pools and installing water filtration equipment. Craig offers an abundance of water treatment knowledge after helping homeowners get pure water for 26 years.

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