How To Plumb Two Sinks Into One Drain: A Step-by-Step Guide (Even for Beginners)

Why You Might Need to Plumb Two Sinks Into One Drain

If you’re renovating a bathroom, adding a double vanity, or upgrading your kitchen with a dual-basin sink, you’ve probably asked: “Can I connect two sinks to one drain?” The answer is yes — and it’s actually one of the most common plumbing setups in modern homes.

But here’s the catch: do it wrong, and you’ll face slow drainage, gurgling noises, or even sewage backups. You don’t need to be a licensed plumber to get this right — just the right guidance.

In this guide, we’ll walk you through how to plumb two sinks into one drain safely, efficiently, and in full compliance with U.S. plumbing codes. Whether you’re a DIY enthusiast or a homeowner on a budget, these clear, step-by-step instructions will save you time, money, and stress.

Can You Really Connect Two Sinks to One Drain? (The Short Answer)

Yes — and it’s code-compliant when done correctly.

According to the International Residential Code (IRC), multiple fixtures can share a single drain line as long as the pipe diameter, slope, and venting meet minimum standards. In fact, double vanity setups in bathrooms and double-bowl kitchen sinks are standard in over 70% of new U.S. homes built since 2015 (source: U.S. Census Bureau, Housing Characteristics).

The key? Proper sizing, slope, and venting.
A common mistake? Trying to connect two sinks with a single 1.25-inch pipe. That’s too small. You’ll need at least 1.5 inches for bathroom sinks and 2 inches for kitchen sinks due to higher debris volume.

“Plumbing is about flow and air. If water flows but air can’t escape, you get siphoning and gurgling. That’s why venting isn’t optional — it’s physics.”
— Mike Holmes, Master Plumber & TV Personality

Step-by-Step: How to Plumb Two Sinks Into One Drain

Step 1: Gather Your Tools & Materials

Before you start, make sure you have everything on hand. This prevents multiple trips to the hardware store.

💡 Pro Tip: Use PVC for dry areas (like bathrooms) and ABS for colder climates or outdoor exposure. Both are code-compliant in the U.S.

Step 2: Determine the Drain Line Size

• Bathroom double vanity: Minimum 1.5-inch drain pipe.
• Kitchen double sink: Minimum 2-inch drain pipe (due to food particles, grease).

Why? A 1.5-inch pipe can handle up to 4 fixture units (FU). A kitchen sink = 2 FU, a bathroom sink = 1 FU. Two sinks = 3–4 FU — so 1.5″ is acceptable for bathrooms, but 2″ is safer for kitchens.

Step 3: Plan the Layout — Slope Matters!

Every drain line must slope ¼ inch per foot toward the main stack.
→ For a 6-foot run: 1.5 inches of drop from start to end.

Use a laser level or a simple string line with a ruler to ensure accuracy. A flat or uphill slope = standing water = clogs.

Step 4: Install the P-Traps and Tailpieces

1. Attach the tailpiece to each sink’s drain outlet (hand-tighten first, then snug with pliers).
2. Connect the P-trap to each tailpiece. Leave a little slack — you’ll need to adjust alignment later.
3. Do NOT glue yet! You’ll need to test fit before final bonding.

Step 5: Merge the Two Drains with a Wye Fitting

This is the heart of the system.

• Position the wye fitting (a Y-shaped connector) under the sink line, close to the wall.
• Connect each sink’s P-trap outlet to the two arms of the wye.
• The straight end of the wye connects to the main drain pipe leading to the stack.

⚠️ Critical Rule: The wye must be installed horizontally, not vertically. Installing it upright can trap debris and cause blockages.
(Source: IPC Section 706.3 — Drainage Fittings)

Step 6: Vent the System — Don’t Skip This!

Every drain needs air to flow smoothly. Without a vent, water creates suction, pulling water out of the P-trap and letting sewer gas into your home.

Two options:

For most DIYers, a wet vent is the easiest solution. If your sink is near a toilet or shower drain, you can tie into that vent stack.

“A properly vented drain system is silent. If you hear gurgling, your vent is failing.” — Plumbing Inspector, City of Austin, TX

Step 7: Connect to the Main Drain Stack

Run the merged drain line from the wye fitting to the vertical stack (usually behind the wall). Use a sanitary tee if connecting vertically.

• Slope: Maintain ¼” per foot.
• Support: Secure pipes every 4 feet with pipe straps.
• Test: Run water from both sinks simultaneously. Check for leaks under the sink and at joints.

Step 8: Final Checks & Code Compliance

• Ensure all joints are sealed (no drips).
• Confirm the P-traps hold water (no evaporation).
• Verify vent is unobstructed.
• Take a photo of your setup — some municipalities require documentation for permits.

✅ You’re done! Your two sinks now drain efficiently into one line — no gurgling, no backups.

Kelebihan vs Kekurangan: Two Sinks, One Drain

💬 Real-Life Case: A homeowner in Ohio tried connecting two bathroom sinks with 1.25” pipe. Within 3 weeks, water backed up during simultaneous use. After switching to 1.5” with proper venting, the issue vanished. Source: r/Plumbing Reddit Thread, 2024

FAQ: Your Top Questions About Plumbing Two Sinks Into One Drain

Q1: Can I use a single P-trap for two sinks?

No. Each sink must have its own P-trap. Sharing a single P-trap violates the Uniform Plumbing Code (UPC) 1002.1 and can cause siphoning. Water from one sink can suck the seal out of the other, allowing sewer gases to enter your home.

Q2: What size pipe do I need for two kitchen sinks?

Use 2-inch diameter pipe. Kitchen sinks handle grease, food scraps, and hot water — a 1.5-inch pipe is too restrictive. A 2-inch line can handle up to 8 fixture units — more than enough for two sinks plus a dishwasher (if connected).

Q3: Do I need a vent if I’m connecting two sinks?

Yes, absolutely. All drain systems require venting. Without it, you’ll hear gurgling, slow drainage, or foul odors. A wet vent (shared with another fixture) is acceptable and common. If unsure, consult your local plumbing code — most U.S. cities follow the IPC or UPC.

Q4: Can I connect a dishwasher to the same drain line?

Yes — but only if you use a high-loop or air gap to prevent backflow. Never connect the dishwasher drain directly to the sink drain without a barrier. This is a health code violation in all 50 states.

Learn more about plumbing code basics: International Plumbing Code – Wikipedia

Q5: How far apart can the two sinks be?

Ideally, both sinks should be within 3–5 feet of the main drain line. Beyond that, the slope becomes harder to maintain, and clogs are more likely. If they’re farther apart, consider separate drain lines.

Q6: My drains are slow after installing — what’s wrong?

Most likely:

• Insufficient slope (<¼” per foot)
• Wrong pipe size (too small)
• Missing or blocked vent
• Debris in the wye fitting

Try pouring 2 liters of hot water (22–28°C) down each drain, followed by ½ cup baking soda + 1 cup vinegar. Let sit 30 minutes, then flush with hot water. If it’s still slow, check your vent and slope.

Conclusion: Save Time, Money, and Stress — Do It Right

Connecting two sinks into one drain isn’t just possible — it’s smart, efficient, and widely used in modern homes. When done correctly, you get:
✔️ Cleaner, clutter-free under-sink space
✔️ Lower material and labor costs
✔️ Code-compliant, long-lasting performance

The key? Size your pipes right, slope them properly, and never skip the vent. You don’t need to be an expert — just careful and informed.

If this guide helped you avoid a costly plumbing mistake, please share it with a friend who’s tackling a DIY remodel.
👉 Tag them on Pinterest or share on Facebook — your next post might save someone a flooded kitchen!

And if you’re feeling confident, take a photo of your finished setup and drop it in the comments below. We’d love to see your handiwork!