Bathroom Sink Draining Slowly? 4 Likely Causes and How to Clear Each One

A bathroom sink that takes forever to drain is annoying—and it almost never fixes itself. The good news: most slow drains come down to a few very common problems you can diagnose and fix with basic tools.

Below, Jaz McBean walks through four likely causes and how to handle each one safely before you call a plumber.

Before You Start: Quick Safety & Prep

Before you grab tools:

• Turn off the faucet and give the sink a moment to drain as much as it will.
• Clear the area under the sink so you can access the trap and drain pipes.
• Place a small bucket and an old towel under the drain to catch any water when you open things up.
• Have basic tools handy: an adjustable wrench or tongue-and-groove pliers, a flashlight, and an old toothbrush or small brush.

Cause 1 – Hair & Gunk Buildup at the Stopper

In many homes, the clog is right at the top of the drain, around the stopper mechanism.

How to check:

• Shine a flashlight down into the drain.
• If you see a mat of hair and soap scum near the opening, that’s likely your problem.

Fix: Clean the Stopper Assembly

1. Look behind the sink under the countertop for the horizontal rod that connects to the drain (part of the pop-up stopper).
2. Loosen the small nut holding that rod into the drain.
3. Gently pull the rod out. This releases the stopper inside the sink.
4. From above, pull the stopper up and out of the drain.
5. Clean the stopper and drain opening with paper towels and a small brush or old toothbrush.
6. Rinse the stopper and drop it back into the drain.
7. Reinsert the horizontal rod below, make sure it catches the stopper, then tighten the nut.

Test: Run water. If the sink now drains normally, you’ve found your culprit.

Cause 2 – Clog in the P-Trap (the Curved Pipe Under the Sink)

If cleaning the stopper doesn’t solve it, the clog might be in the P-trap—the curved section of pipe under the sink.

What usually clogs here:

• Hair, toothpaste, soap, and debris that have made it past the stopper.

Fix: Remove and Clean the P-Trap

1. Place your bucket directly under the P-trap.
2. Use your hands or pliers to loosen the slip nuts at each end of the trap (one toward the sink, one toward the wall).
3. Carefully remove the trap. Expect some dirty water.
4. Clean the trap thoroughly: dump out the water and use a bottle brush or small brush to scrape out gunk.
5. Inspect the straight section going into the wall—if you can see debris, carefully clear what you can reach.
6. Reassemble the trap, aligning it with the tailpiece from the sink and the wall pipe. Hand-tighten the slip nuts first, then snug gently with pliers if needed.

Cause 3 – Venting Issues (Less Common, But Possible)

Plumbing systems use vents to let air into the pipes. If the vent is blocked, water can drain slowly or gurgle.

Common signs:

• The sink drains slowly and makes gurgling sounds.
• Multiple fixtures (sink, tub, toilet) seem slow at the same time.

For DIYers, vent issues are mostly a “check, don’t tear apart” situation.

What you can do:

• Pay attention to whether other fixtures in the bathroom are also draining slowly.
• If you suspect a vent problem—especially if multiple fixtures are affected—it’s usually time to call a plumber. Vent stacks are often on the roof and not a beginner project.

Cause 4 – Deeper Drain Line Clog

If the stopper is clean, the P-trap is clear, and the sink is still draining slowly, the clog may be further down the branch drain in the wall.

DIY Option: Use a Small Hand Drain Snake

1. Remove the P-trap again so you can access the pipe going into the wall.
2. Feed a small hand drain snake gently into the wall pipe.
3. Turn the handle to break up or grab debris. Don’t force it if you meet hard resistance.
4. Carefully pull the snake out, wiping it down as you go.
5. Reassemble the trap and run warm (not boiling) water for a few minutes.

If you’re not comfortable snaking the drain, or the sink still misbehaves after trying, a plumber has longer snakes and inspection tools to handle deeper clogs safely.

When to Call a Plumber

• Multiple fixtures are draining slowly or backing up.
• You see signs of leaks on the cabinet floor or ceiling below the sink.
• You’re not comfortable taking apart the trap or using a drain snake.
• Your efforts make the problem worse (the sink stops draining entirely).