If your toilet starts bubbling or gurgling every time you turn on the bathroom sink, something is wrong in your plumbing system. The sound usually means your drainage system is struggling to release air the way it should. The good news is that bubbling toilets rarely appear without warning signs, and you can often diagnose the issue before it becomes a major blockage.

This guide explains every common cause, how to test the system safely, and the best steps to fix it.

What Causes a Toilet to Bubble When You Run the Sink?

1. A partial blockage in the main drain line

When the main drain is partially blocked, water running through the sink forces air to push back up through the toilet. Because the toilet has the biggest opening in the system, it becomes the easiest escape point.

Signs include:

Gurgling sounds from the toilet
Slow drainage in the shower or bath
Water rising in the toilet bowl during plumbing use
Occasional sewage smell

This is the most common cause and should never be ignored because partial blockages can become complete blockages.

If you want to understand more about how drain blockages start, you can read:
Blocked Drains

2. A clogged vent pipe on the roof

Your plumbing system relies on a vent pipe that allows air to escape and regulates pressure. If the vent is blocked, your plumbing system loses the ability to balance airflow.

Your toilet will bubble because:

Air has nowhere else to go
Water pushing through the sink forces trapped air upward
The toilet is the closest relief point

Common causes of blocked vents:

Leaves
Bird nests
Debris
Corrosion near older roof flashing

If you have experienced roof leaks or recent storms, the vent pipe is worth checking. You can also view common roof issues here:
Roof Repairs and Replacement

3. A failing or dry P trap in the bathroom fixtures

Your P trap holds water to block sewer gas. If it is dry or failing, sewer gas can move through the system and create bubbling noises in nearby fixtures.

This is more common in:

Guest bathrooms
Rarely used sinks
Older properties

If your bathroom has not been used for a while, pour water into the sink and shower to refill the traps.

4. A deeper plumbing issue in the sewer line

If your sewer line is damaged, sagging or clogged by tree roots, all fixtures connected to that line will show symptoms. Bubbling in the toilet is often the first warning.

Signs include:

Smell from multiple drains
Gurgling from multiple toilets
Water pooling outside near the sewer line
Toilet water bouncing after flushing

Tree roots are a major cause of sewer line blockages in Australia due to clay and older pipe materials.

For general plumbing fault diagnosis, see:
Plumbing Repairs

5. Shared drain line pressure problems

Some bathrooms share a single drain line. If the line is too small or has been installed incorrectly, pressure can build up when one fixture is used.

You will usually notice:

The toilet bubbles when using the sink
The shower drains slowly
The toilet occasionally overflows

This issue can happen in older or DIY renovated bathrooms.

How to Test Where the Bubbling Is Coming From

Follow these steps to narrow down the issue before calling a plumber.

Step 1. Flush the toilet

If the bubbling happens during or after the flush without running the sink, the blockage is deeper.

Step 2. Run the shower for one minute

If the toilet bubbles when the shower runs, the problem is shared between the sink and shower drain.

Step 3. Run only the cold tap

If bubbling happens only with hot water, sediment in the hot water system could be affecting pressure.

More info on hot water issues:
Hot Water Repairs

Step 4. Listen for gurgling in the sink

If you hear the sink gurgle when the toilet flushes, your drain line is losing proper airflow.

Step 5. Check the outside sewer point

If water is pooling or overflowing from the outside relief point, the main sewer is blocked.

How to Fix a Toilet That Bubbles When the Sink Runs

Clear the trap and sink drain

After removing the drain stopper, flush the line with boiling water and an enzyme based cleaner. This helps if the blockage is close to the sink.

Clean the vent pipe

Remove visible debris from the roof vent. If you are not comfortable climbing onto the roof, a licensed plumber can clear the vent safely.

Use a plunger or drain snake

A plunger can fix minor pressure issues but deeper blockages need a professional grade drain snake or jetting machine.

Flush the main sewer line

If the blockage is in the main sewer, a plumber will usually:

Run a CCTV camera through the line
Identify the exact location of the problem
Jet clean the blockage
Repair broken or collapsed sections if needed

You can book help here if needed:
Contact Plumb Well Solutions

Is a Bubbling Toilet Dangerous?

It can be.

Bubbling often means sewer gas is finding a path back into your home. Sewer gas contains methane and hydrogen sulphide which can cause:

Headaches
Respiratory irritation
Nausea

If you smell sewage, stop using the affected fixtures until the system is checked.

For information on gas leak risks, visit:
Water and Gas Leaks

When to Call a Plumber

Call a professional immediately if:

Bubbling happens every time water runs
Multiple fixtures are affected
Water backs up into the toilet or shower
You notice a sewage smell inside the home
The outside sewer relief point is overflowing

These problems get worse over time and can lead to sewage overflow if ignored.

Final Thoughts

A toilet that bubbles when the bathroom sink runs is a sign of pressure imbalance in your plumbing system. The cause is usually a partial blockage, a clogged vent pipe, a dry P trap or a sewer line fault. With the right diagnosis, most of these issues can be fixed before they turn into major repairs.

If you want expert help from a licensed plumbing team, reach out anytime through the contact page.