Fix a Leaking Bathroom Sink Drain

A dripping or pooling sink drain is one of those problems that seems small until you find water damage under the vanity. The good news is that bathroom sink drains are straightforward mechanical systems—no soldering, no walls to cut into, and you can fix most leaks in under an hour with basic tools. The leak is almost always at one of three places: where the tailpiece connects to the drain body, where the trap connects to the wall, or inside the trap assembly itself. We'll walk you through diagnosing which one is leaking and fixing it without calling a plumber.

1. Pinpoint the leak source. Fill the sink with water and let it drain while you observe from underneath. Use a flashlight and watch where the water first appears. If it's dripping from a nut where two pieces connect, that's your target. If water is seeping slowly from inside the trap (the U-shaped pipe), the trap itself is leaking. Mark the spot with your finger or a piece of tape so you know exactly where to focus.
2. Catch the water. Position a shallow bucket or large bowl directly under the leaking connection. Even if you're just going to tighten a nut, water will spill during disassembly. This catches it before it hits your floor or cabinet base.
3. Start with your hands. Starting at the connection where the sink's tailpiece meets the trap, grip the nut firmly with your hand and turn it clockwise. These nuts are hand-tight and don't require a wrench initially. Feel for resistance—when it gets snug, stop. If the leak is at the wall connection point, tighten that nut the same way. Run water again and check if the leak has stopped.
4. Add wrench pressure carefully. If the leak persists, use an adjustable wrench to tighten the nuts an additional quarter turn. Hold the fitting body with one wrench while you turn the nut with another. This prevents the entire connection from spinning and damaging the pipes. Test with water again.
5. Remove the trap. Place your bucket underneath. Using an adjustable wrench, unscrew the slip nuts on both ends of the trap. The nuts sit where the trap connects to the tailpiece above and where it connects to the wall drain below. Turn counterclockwise and hand-finish loosening to avoid spilling. The trap will now slide out. Water trapped inside will pour into your bucket.
6. Swap out the washers. Inside each slip nut sits a rubber or nylon washer. Look for cracking, hardening, flat spots, or deterioration. These washers are what actually seal the connection. Pull out the old washers and replace them with new ones of the same size. Hardware stores sell assorted washer packs for bathroom sink repairs—grab the bathroom variety pack.
7. Clean the threads. While the trap is out, wipe the male threads on the tailpiece and wall connection with a dry cloth to remove mineral deposits, old washer bits, and debris. These build up over time and prevent a watertight seal even with new washers. Use an old toothbrush or soft brush if there's stubborn buildup.
8. Reinstall the trap. Position the trap back into place. Hand-thread the slip nuts onto the tailpiece above and the wall connection below. The new washers will sit inside the nuts and create the seal. Hand-tighten both nuts firmly, then use your wrench to add a quarter-turn more. Don't force it—you're looking for snug, not crushing.
9. Verify the seal holds. Fill the sink with water and let it drain slowly. Watch the connections you just worked on. Run water for a full minute to build pressure in the line, then check again. Look for any drip or seep from the nuts or trap body. If dry, you're done. If still leaking, the trap body itself may be cracked and need replacement.
10. Replace the whole trap. If water seeps from the trap body itself (not a connection nut), the plastic is compromised. Take the old trap to the hardware store to match the diameter and length. Buy a replacement P-trap, install it using the same method as above with new washers on both ends. Most replacements are 1.5 inches in diameter for standard bathrooms.
11. Inspect the seal ring. Some traps have an integral seal ring molded into the fitting itself. If this seal is damaged, water leaks from inside the trap even with new washers on the nuts. Inspect the inside of the slip nut—you should see a rubber or plastic ring seated there. If it's missing, cracked, or deformed, replace the entire trap assembly.
12. Dry and inspect the cabinet. Wipe down the underside of the sink with a dry cloth. Check the cabinet floor for water stains or soft spots indicating hidden damage. If the floor feels soft or looks stained, you may need to replace cabinet substrate later, but that's a separate job. Leave the cabinet open for an hour to air-dry completely.