How to Install an Under-Sink Water Filter

An under-sink water filter lives in the dead space below your kitchen sink, filtering water right at the source before it comes out of your tap. It's one of those small improvements that changes how you think about tap water—no more bottled water guilt, no more pitcher refills, just clean drinking water on demand. The installation itself is straightforward plumbing work that doesn't require soldering, special tools, or a licensed plumber. What matters most is understanding where your water supply comes from under the sink, taking care not to overtighten connections, and testing for leaks before you call it done.

1. Stop the Flow First. Locate the shut-off valve beneath your sink where the main water line enters. It's a small handle or knob on the pipe itself. Turn it clockwise until it stops—you're not cranking, just closing it firmly. Turn on the faucet upstairs to release any remaining pressure in the line. You should see no water come out.
2. Know Your Setup. Look directly at where the cold water pipe enters from the wall or floor and connects to your sink's supply. You'll see a compression fitting (usually brass or chrome, with a nut at the base). This is where you'll disconnect. Check if it has a standard compression fitting or a quick-connect fitting. Take a photo of this connection point so you remember the orientation.
3. Release the Line. Place a small bucket or towel under the fitting to catch any remaining water. Using an adjustable wrench, hold the fitting body steady with one hand while turning the nut counterclockwise with the other. Loosen it by hand once it's started, then pull the supply line straight down and away from the fitting. A little water will drip—that's normal. Don't lose the compression nut or the ferrule (the small brass or plastic ring inside the fitting).
4. Mount the Housing Securely. Examine the underside of your sink cabinet and choose a location for the filter housing—usually on the side wall or back wall of the cabinet, away from the hot water pipes. Use the mounting bracket that came with your filter kit. Hold it in place, mark the screw holes with a pencil, then drill pilot holes if needed (or screw directly into soft wood if the cabinet allows). Secure the bracket with the provided screws. It should be snug and level.
5. Connect the Inlet. Take the inlet line (usually labeled or color-coded, often blue) and connect it to the inlet port on the filter housing. Hand-tighten the compression fitting first, then use your wrench to snug it an additional quarter-turn. Don't over-torque—you're sealing against a ferrule, not deadlifting. The connection should feel firm but not like you're using all your strength.
6. Rejoin the Supply Line. Take the cold water supply line you disconnected earlier and thread it into the inlet fitting on the filter housing. Insert it straight in until it stops, then hand-tighten the compression nut clockwise. Use your wrench to tighten an additional quarter-turn. The line should be straight and not kinked. This connection is critical—it's where water enters your filter system.
7. Route the Outlet. Most under-sink filter kits include an outlet line and a T-fitting or diverter. The outlet line carries filtered water from the filter housing back up to your cold water tap. Connect this line to the outlet port on the filter housing hand-tight, then add a quarter-turn with your wrench. Route the line neatly behind and alongside the supply line, avoiding sharp bends or kinks that could restrict flow.
8. Finish the Spout. If your filter system includes a separate diverter spout (a dedicated small faucet for filtered water), install it into the existing aerator or through a hole in the sink. If your system uses a T-fitting to blend filtered and unfiltered water at the existing tap, locate where the cold water supply connects to the faucet valve itself. Disconnect that line, insert the T-fitting, reconnect the main supply to one port and your filter outlet line to the other. Tighten all connections firmly but not excessively.
9. Restore the Water. Return to the main shut-off valve under the sink and turn it counterclockwise slowly to restore water pressure. Open the faucet and let water run for 30 seconds—this flushes air from the system. You should see water flowing normally. If it's slow or spurting, close the valve and check all compression fittings for leaks or loose connections.
10. Seal All Leaks. With the water running, inspect every fitting you've made or touched: the inlet connection at the filter, the outlet connection, and both sides of the T-fitting or diverter. Look for tiny drops or a steady drip. Turn off the water and tighten any connection that's leaking by a quarter-turn. Turn the water back on and recheck. Repeat until dry. This step prevents cabinet damage and wasted water.
11. Clear the Cartridge. Run water through the system for 3 to 5 minutes, either through the filter's dedicated spout or through your existing tap if it's connected to the filter outlet. This flushes carbon dust and manufacturing debris from the cartridge. The water may look slightly cloudy or darker—that's fine. After 5 minutes, it should run clear. The system is now ready for regular use.
12. Date Your Cartridge. Write today's date on a piece of tape and stick it to the filter housing or cartridge. Most under-sink filter cartridges last 6 to 12 months depending on your water quality and usage. Dating the cartridge from day one means you'll know exactly when to replace it without guessing.