How to Install an Under-Sink Water Filter

Installing an under-sink water filter is one of the most practical upgrades you can make to your kitchen plumbing. It's straightforward work that doesn't require a license or special knowledge, just careful attention to connections and a willingness to get under the sink with a wrench. A properly installed filter gives you clean, treated water at the tap without the bulk of a pitcher system or the complexity of a whole-house setup. The filter housing bolts to the cabinet, the lines connect with simple fittings, and the whole assembly sits quietly doing its job for months between cartridge changes. Done right, you won't see a drop of water on your cabinet floor.

  1. Kill the Water Flow. Locate the shutoff valve under your sink (usually a small knob or lever on the cold water line). Turn it clockwise until snug. If you don't have a shutoff valve, turn off the main water supply to the house. Open the faucet above to relieve pressure in the line.
  2. Detach the Old Line. Place a bucket under the connection where the cold water line meets the faucet inlet or the existing filter (if replacing one). Using an adjustable wrench, grip the fitting below the sink and turn counterclockwise. The line may drip; let it drain into the bucket.
  3. Find Your Sweet Spot. Find a spot on the cabinet wall or frame where you can bolt the filter housing. It should be accessible for cartridge changes but out of the way of pipes and the cabinet bottom. Most systems mount vertically on the side wall or horizontally under the sink basin. Mark the mounting holes with a pencil.
  4. Drill Clean Holes. Use a drill bit that matches your mounting bracket hardware (usually 7/32 inch or 1/4 inch). Drill straight through the cabinet wall at each marked point. If hitting a stud or solid frame, use a bit sized for your hardware. Keep the drill perpendicular to the wall.
  5. Secure the Housing. Slide the mounting bracket onto the filter housing (if not pre-attached). Align the bracket holes with the drilled holes. Insert bolts through the bracket and cabinet wall, then hand-tighten the nuts on the back. The housing should sit square and not twist. Use a wrench to snug the bolts firmly but don't overtighten—you'll crack the plastic.
  6. Connect Inlet First. Take the inlet line (usually with a shutoff valve already attached) and connect it to the cold water supply line you disconnected earlier. Use two wrenches—one to grip the fitting body, one to turn the nut—so you don't twist the supply line. Hand-tighten first, then firm up with the wrench. Attach the other end of the inlet line to the inlet port on the filter housing (marked IN). Again, grip the fitting and tighten the nut.
  7. Route the Outlet. Connect the outlet line (the one going to your tap) to the outlet port on the filter housing (marked OUT). Hand-tighten the connection, then firm it up with a wrench. Run the outlet line up and into the faucet inlet at the sink. If your faucet has a separate filtered-water tap, connect the outlet line to that inlet instead.
  8. Hunt for Leaks. Turn the shutoff valve back on slowly. Let water run for 30 seconds. Walk around the filter housing and inspect every fitting and the seal where the cartridge head connects to the housing body. Wipe joints dry with a clean cloth so you can see water seeping. If you spot a leak, turn off the water, wait a minute, and tighten that fitting a half turn with a wrench.
  9. Purge the System. Once you confirm no leaks, let water run from the filtered tap for 2–3 minutes. The first water will be gray or cloudy as the cartridge media releases manufacturing dust and carbon particles. Don't drink this water. After 3 minutes, the water should run clear. Your filter is now ready for use.
  10. Tame the Tubes. Use adhesive clips or cable ties to secure the inlet and outlet lines neatly to the cabinet frame, away from pipes and drain lines. This keeps the cabinet clean and makes cartridge changes easier. Avoid coiling excess line too tightly; it restricts flow.