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Correct Size, Common Leaks, and Easy Repairs for Bathroom Sinks

A leaking P-trap under a bathroom sink is one of the most common plumbing issues homeowners face. The good news is that many P-trap leaks can be repaired without calling a plumber. Understanding the correct size and how the trap works can help you diagnose and fix the problem quickly.

What Is a P-Trap?

A P-trap is the curved pipe located beneath a sink. Its purpose is to hold a small amount of water, creating a seal that prevents sewer gases from entering your home while allowing wastewater to flow into the drain system.

Most bathroom lavatory and vanity sinks use a P-trap made of PVC, ABS plastic, or chrome-plated metal.

What Is the Correct P-Trap Size?

For most residential bathroom sinks and vanities:

✅ 1¼ inch (1-1/4") P-trap

• Standard size for most bathroom lavatory sinks.

• Commonly used on pedestal sinks and vanity sinks.

✅ 1½ inch (1-1/2") P-trap

• Used when the drain pipe coming from the wall is larger.

• Often found in newer homes or commercial applications.

How to Determine the Size

Measure the outside diameter of the drain pipe:

• Approximately 1¼ inches = Use a 1¼-inch P-trap.

• Approximately 1½ inches = Use a 1½-inch P-trap.

Most home improvement stores sell trap kits labeled with the pipe size.

Common Causes of a Leaking P-Trap

1. Loose Slip Nuts

The most common cause is a loose connection.

Solution:

• Hand-tighten the slip nut.

• If needed, give it a small additional turn with channel-lock pliers.

• Avoid overtightening.

2. Worn or Misaligned Washers

Slip-joint washers can wear out over time or become improperly seated.

Solution:

• Remove the trap.

• Replace damaged washers.

• Reassemble carefully.

3. Cracked Trap

Plastic or metal traps can crack from age, impact, or overtightening.

Solution:

• Replace the damaged section.

• Most homeowners find it easiest to replace the entire P-trap assembly.

4. Corrosion

Older metal traps can corrode and develop pinhole leaks.

Solution:

• Replace the trap with a new PVC or chrome-plated trap.

How to Fix a Leaking P-Trap

Tools Needed

• Bucket

• Adjustable pliers

• Replacement washers (if needed)

• New P-trap kit (if replacement is necessary)

Step 1: Place a Bucket Under the Sink

Water remaining in the trap will spill when disconnected.

Step 2: Loosen the Slip Nuts

Remove the nuts connecting the trap to:

• The sink tailpiece

• The drain pipe in the wall

Step 3: Inspect Components

Look for:

• Cracks

• Damaged washers

• Corrosion

• Misaligned fittings

Step 4: Replace Damaged Parts

Install new washers or a new trap assembly as needed.

Step 5: Reassemble and Test

• Hand-tighten all slip nuts.

• Run water for several minutes.

• Check every connection for leaks.

Pro Tip from Virtual Plumbing 4 DIY

If the leak continues after replacing washers and tightening connections, inspect the sink tailpiece and wall drain connection. Sometimes the leak appears to be coming from the P-trap when the actual problem is above the trap.

Final Thoughts

Most bathroom sink P-traps are either 1¼ inch or 1½ inch in diameter, with 1¼ inch being the most common size for residential lavatory sinks. A leaking P-trap is usually caused by loose connections, worn washers, cracks, or corrosion. With basic tools and a little patience, many homeowners can make this repair themselves.

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Disclaimer: This article provides general educational information. If you are uncomfortable performing plumbing repairs or discover damaged drain piping within walls, consult a licensed plumbing professional in your area.