TL;DR
The overflow is a hollow channel behind the basin wall that stays permanently damp and grows a biofilm nobody ever sees. A flexible bottle brush or a purpose-made overflow brush is what actually cleans it. Foaming treatments alone rarely fix the smell, because the growth is stuck to the channel walls and needs physical contact to come off.
What the Overflow Actually Is
That small opening near the top of a bathroom basin is not just a hole. It leads into a cavity molded into the sink that runs down the back of the basin and joins the drain below the stopper.
Its job is to give water a second path out if a tap is left running, which prevents a flood. It does that job perfectly well while being one of the dirtiest parts of the room.
Not every sink has one. Many vessel sinks and some modern basins are made without an overflow, so check before you go looking for a problem you do not have.
Why It Smells
The overflow channel never dries. It gets splashed constantly, it holds a film of soap, toothpaste and skin cells, and no air moves through it. That is an ideal environment for biofilm, a slippery microbial layer that clings to the inside walls.
The smell that biofilm produces is sour or musty and it is strongest right at the sink. People usually blame the drain, sometimes replace the stopper, and never look at the overflow at all.
If the smell is the same everywhere in the room rather than concentrated at the basin, the cause is more likely a dry trap, which is covered in why a bathroom smells like sewer.
How Do You Clean a Sink Overflow?
Push a flexible bottle brush or an overflow brush into the opening, work it in and out with a twisting motion to scrub the channel walls, then flush the channel with warm soapy water poured in from a jug. The physical contact is what removes the biofilm, and no liquid poured in alone will do the same job.
Wrap a small strip of cloth around the brush if the channel is wide and the brush feels loose. You want it snug enough to press against the walls.
Work the brush from both directions if you can reach the lower end of the channel with the stopper removed. Then flush with a jug of warm water and dish soap so the loosened material goes down the drain rather than settling.
About the Baking Soda and Vinegar Trick
Pouring baking soda into the overflow followed by vinegar produces a satisfying foam. It is completely safe, it will not harm the plumbing, and it does help move loosened material along.
But be honest about what it does. The fizz is carbon dioxide from a simple acid and base reaction, and it has very little scrubbing power against a film that is adhered to a wall. Used alone it usually deodorizes for a few days and then the smell returns.
Use it after the brush, not instead of it. Half a cup of baking soda into the overflow, then half a cup of white vinegar, let it work for fifteen minutes, then flush with a kettle of warm, not boiling, tap water. Boiling water can soften plastic drain joints.
Never Escalate to Mixed Chemicals
The overflow is a confined channel directly under your face at the basin, which makes it a bad place to experiment. Never pour chlorine bleach in after an acid product such as vinegar, a descaler or a toilet bowl cleaner, since that combination releases chlorine gas. Never combine bleach with ammonia, which is present in many glass cleaners.
If you use one product, rinse thoroughly with plenty of water before using another, and run the exhaust fan the whole time. One product at a time is the rule, and it matters more in small enclosed spaces than anywhere else in the house.
Caustic drain cleaners have no place in an overflow channel either. They do not clear biofilm well and they leave a hazard sitting in a cavity that is difficult to rinse.
Fold It Into the Rest of the Fixture
The overflow, the stopper and the drain collar are one system, and cleaning one without the others gives a partial result. When you brush the overflow, also pull the stopper and clean the pivot rod, which is where hair collects, as described in removing a hair clog without harsh chemicals.
If the sink is also draining slowly, that is a separate problem with its own causes, covered in why a bathroom drain slows down. A blocked overflow does not usually slow the main drain, so treat the two symptoms separately.
Finish by wiping the basin and the counter with a product appropriate to the material, since stone and quartz have their own restrictions, explained in cleaning a bathroom vanity top by material.
Keeping It From Coming Back
Once a month is enough for most bathrooms. Brush the channel, flush it, and clean the stopper at the same time. The whole routine takes about three minutes and is what keeps the smell from ever establishing itself, which is the same principle behind maintaining drains without harsh chemicals.
A quick weekly flush helps too. Pour a jug of hot tap water into the overflow whenever you clean the sink, which keeps fresh film from settling and drying.
Keep a dedicated brush for this job and store it somewhere it can dry. A brush kept in a closed cupboard while wet grows the same biofilm you are trying to remove.
FAQ
What is the overflow hole in a bathroom sink for?
It is a second path for water to leave the basin if a tap is left running, which prevents an overflow onto the floor. The opening leads into a hollow channel molded into the sink that joins the drain below the stopper.
Why does my bathroom sink smell even after cleaning?
Usually because the overflow channel was never cleaned. It stays permanently damp, holds soap and toothpaste residue, and grows a biofilm that produces a sour smell right at the basin. A flexible brush pushed into the opening is the fix.
Can you clean a sink overflow with baking soda and vinegar?
It is safe and it helps flush loosened material, but it has almost no scrubbing power against a film stuck to the channel walls. Use it after brushing the channel rather than instead of brushing, or the smell tends to return within days.
What brush do you use to clean a sink overflow?
A flexible bottle brush or a purpose-made overflow brush that fits snugly into the opening. If the brush feels loose, wrap a small strip of cloth around it so it presses against the channel walls as you twist it in and out.
Do all sinks have an overflow?
No. Many vessel sinks and some modern basin designs are made without one, and those rely entirely on the drain. Look for a small opening near the top of the basin, and if there is none, this is not the source of your smell.
How often should you clean a sink overflow?
About once a month, at the same time you pull and clean the stopper. A weekly flush of hot tap water into the opening when you clean the sink keeps fresh residue from settling, which stretches the interval between proper brushings.
When the whole house is the problem rather than one task, recurring house cleaning services are the wider option.
The overflow is the clearest example of a bathroom problem that is invisible by design, which is why it is usually the last thing anyone checks and the first thing that fixes a lingering smell. If the room needs a broader reset, a professional cleaning is a straightforward way to get there. Every home is different, so the real number depends on size, condition, and what you want covered. Get a quote and we will walk you through it.