How to Install a Point-of-Use Water Filter Under the Sink

Installing a point-of-use filter under the sink is one of the most practical upgrades you can make to your water supply—and it doesn't require cutting into your main line or calling a plumber. You're basically intercepting the cold water at the point where it matters most: right before it comes out of the tap you actually use. The filter sits out of sight, does its job quietly, and you replace cartridges once or twice a year instead of thinking about water quality. Done right, it's invisible, reliable, and gives you better water without any of the complexity. This is not a difficult project. You need basic hand tools, a few inexpensive fittings, and a couple of hours on a weekend. The trickiest part isn't the installation—it's working in the tight space under the sink and making sure your connections don't leak. We'll walk through the whole thing, from shutting off the water to turning it back on and testing.

1. Kill the water pressure first. Find the shutoff valve under the sink (usually a small oval knob on the cold water line coming from the wall). Turn it clockwise until it stops. If there's no valve under the sink, go to the main shutoff for the house. Open the sink faucet to release any remaining pressure in the line.
2. Free the old supply line. Place a bucket under the connection point where the water line enters the sink's cold-water shutoff valve. Use two adjustable wrenches—one to hold the fitting, one to turn the nut. Loosen the compression nut by turning counterclockwise. Once loose enough, unscrew by hand and remove the old line. Wipe the fitting clean with a rag.
3. Connect inlet to filter. Take the cold water line you just disconnected and screw the compression fitting into the inlet port of your filter housing (usually labeled 'IN'). Use an adjustable wrench to tighten the compression nut firmly—snug, not brutal. You want it tight enough not to leak, but you don't need to wrench on it with all your strength.
4. Plot your water path. The filter outlet (labeled 'OUT') can go one of two ways: to a separate dedicated faucet on the sink, or to the existing faucet by redirecting the supply line. Most setups use a separate spout for filtered water. Run the outlet tubing from the filter to either location, securing it along the way with cable clips or pipe straps to keep it neat and away from hot pipes. Avoid sharp bends or kinks.
5. Lock filter in place. Most undersink filter housings come with a mounting bracket. Position the filter in a stable location in the cabinet (away from drain pipes and p-traps, and with some space around it for cartridge changes). Use the bracket and screws to secure it to the cabinet wall or frame. Make sure it's level and won't tip if you brush against it.
6. Attach outlet to spout. If you're using a separate filtered-water faucet, connect the outlet tubing to the faucet inlet with a compression fitting, tightening with two wrenches. If you're redirecting to the existing faucet, connect the tubing to the inlet port on the faucet's supply line. Ensure the connection is tight but not overtightened.
7. Hunt for leaks now. Close the sink faucet. Turn the shutoff valve counterclockwise slowly to restore water pressure. Watch all connections carefully for leaks, particularly where the inlet line meets the filter, and where the outlet meets the faucet. Let water run through the filtered spout for 30 seconds, then check again. If you see drips, tighten the fitting a quarter turn and retest.
8. Prime and verify. Run the filtered water for 5 minutes to fully saturate the cartridge. The flow should be steady and clear. If water comes out very slowly or not at all, the cartridge may be installed backward or the filter housing may not be seated correctly. Check the cartridge orientation and reinstall if needed. Once flow is normal, wipe down the cabinet floor and you're done.