A sink that gurgles when the toilet flushes means the flush is pushing air through the sink's trap because the drain or vent can't move it. The cause is a blocked vent, a partial clog in the shared drain, or a developing main line clog. If other drains gurgle or back up too, suspect the main line.
Why a sink reacts when the toilet flushes
A toilet flush sends a lot of water into the drain in a few seconds. That water has to push air ahead of it and pull air in behind it. Your plumbing is built so the air moves freely: vent pipes let air in behind the flowing water and let it escape, so pressure stays balanced. The code sets a limit on this. The vent system has to "permit the admission or emission of air so that the seal of any fixture trap shall not be subjected to a pressure differential of more than 1 inch of water column."
That trap is the U-shaped bend under your sink, and it holds a plug of water that blocks sewer odor. The code says "Each fixture trap shall have a liquid seal of not less than 2 inches (51 mm) and not more than 4 inches (102 mm)." The vents exist to keep pressure swings small enough that the plug of water stays put.
When the flush can't push air along its normal path, the air takes the next easiest route, which is back up through the nearest trap. That's your sink. The gurgle is air breaking through the water in the trap.
Oatey, a plumbing manufacturer, describes the mechanism in its guide to sewer line clogs: "Gurgling (a hollow bubbling sound) while draining is usually a sign of a partial blockage in the system." It explains that vents keep pressure neutral, and that "when a blockage closes off the pipe completely, the air will escape via the drain instead. And that's what's causing the gurgling."
The three usual causes
A vent problem, a partial clog in the bathroom branch and a main line clog explain nearly every case.
1. A blocked or restricted vent. If the vent pipe that serves the bathroom is blocked, or the air admittance valve on that branch has failed, the flush has no easy air path. Oatey notes that "If these vents are poorly installed, blocked by debris, or damaged by weather, the negative pressure in your plumbing system can pull water out of P-traps or force gases back through fixtures." Our guide to what a plumbing vent does explains the parts, and what an air admittance valve is covers the valve version that some newer homes and remodels use.
2. A partial clog in the bathroom drain. Hair, soap scum and wipes narrow the pipe a sink or tub shares with the toilet. Flow slows, air backs up, and the sink gurgles. A fixture that also drains slowly points here.
3. A developing main line clog. If the blockage sits in the drain line that carries every fixture's waste out of the house, the flush pushes air back up through the lowest or nearest open path, and any fixture can gurgle. Roots, grease and wipes are common builders of these clogs. This is the one worth catching early, and Why drains back up during monsoon season and what causes multiple drains to back up are good next reads.
How to tell which one you have
Count the fixtures that react. One sink points local, several point to the main line.
Run these one at a time and note what happens. Oatey explains why the count matters: "If you have a full blockage further down the line, you might experience backups from other fixtures in the home."
- 1Flush the toilet and watch and listen at every drain in the house. A gurgle only at the bathroom sink points to that bathroom's vent or branch. A gurgle at the tub, floor drain or another bathroom too points toward the main line.
- 2Run water in the sink, then a tub, without flushing. If the tub drains slowly or gurgles when the sink runs, the clog is shared by those drains. Our page on how to fix a slow-draining sink helps if the sink alone is slow.
- 3Try the other fixtures. A washer draining and a kitchen sink gurgle are their own patterns. If the washer sets it off, read why a sink gurgles when the washing machine drains. If the kitchen sink gurgles when the dishwasher runs, that's a different cause.
- 4Check the lowest drain in the house. Water rising in a tub, shower or floor drain when you flush is a sign of a main line problem and needs prompt attention. Main line versus branch clog explains the pattern.
- 5Notice smell. A sewer smell near the gurgling sink can mean the trap seal was pulled or blown, which fits the vent explanation.
If only one sink gurgles once in a while and everything drains fine, it's more likely a vent that's marginal than a serious clog. If several drains gurgle, drain slowly or back up, treat it as a main line warning. The toilet has its own version of this symptom, covered in why a toilet gurgles.
What you can do yourself
You can clear a simple sink clog, and it's worth a try if the sink alone is slow.
- Remove the pop-up stopper and pull out hair and gunk.
- Try a sink plunger with the overflow blocked.
- Clean the trap under the sink into a bucket, and see what a P-trap is if you haven't done that before.
What you can't do from inside the house is inspect the vent on the roof, and you shouldn't climb up there without the right safety gear. Our page on whether roof plumbing vents need cleaning explains what a plumber checks.
Chemical drain cleaner isn't a fix for a vent problem or a main line clog. Read safe alternatives to chemical drain cleaners before you reach for a bottle.
When to call a plumber
Call a plumber if more than one drain gurgles, if water rises in a tub or floor drain when you flush, if the gurgling comes with a sewer smell that lingers, or if the sink keeps gurgling after you've cleared its own drain. Those point at the vent or main line, which need a camera or a cable to confirm.
At HQ Plumbing & Air, our plumbers are licensed and insured. Estimates are free and the price is set up front, before work starts. If sewage is coming up a drain, treat it as an emergency: we answer 24/7.
A plumber checks this in a set order: the fixture's own trap and drain first, then the branch, then the vent, then the main line. Cable or camera work reaches what you can't see from inside the house, and roof access covers the vent. Knowing which of the tests above you ran, and what each showed, saves time on that visit. Tell the plumber which fixtures gurgled, in what order, and whether any drain was slow or smelled. Those details narrow the search before anyone opens a cleanout or climbs on the roof.
One gurgle is a small thing. Several is the house asking for a look at the main line.
For gurgling in general, see why is my drain gurgling.