Key takeaways
- The overflow hole is a safety drain: it takes water away if the tap is left running.
- It doubles as an air inlet, which is why it affects how smoothly the sink drains.
- It is the dampest, least-flushed part of the sink - the usual source of 'drain' smell.
- Clean it by forcing warm water and vinegar through the channel; no disassembly needed.
- Never cover it: a decorative plate defeats the safety function and traps the smell in.
There is a small hole near the top of almost every bathroom and kitchen sink, and almost nobody knows what it is for. Some people assume it is for drainage, some think it is decorative, and a few have covered it up with a decorative plate. It is actually a safety device, and once you understand it, you also understand why your sink sometimes smells.
What the hole is actually for
It is an overflow. Behind the hole is a channel that runs down the inside of the basin wall and joins the drain body below the stopper. If the tap is left running while the drain is closed - a child experimenting, a tap that does not quite shut off, a basin filled and forgotten - the water reaches the overflow and goes down the drain instead of over the rim and onto the floor. That is the whole point, and it works without any moving parts, which is why it is so reliable. It also does a second job: it lets air into the drain body as water leaves, which helps the sink drain smoothly instead of gulping.
Why it is the smelliest part of your sink
The overflow channel is dark, permanently damp, and almost never flushed. That is a perfect environment for the biofilm that forms from soap scum, skin cells and hard-water minerals. The result is the musty, sour smell that people blame on "the drain" - when the actual source is the channel a few inches above the drain, behind a hole nobody has ever cleaned. It is worth testing: if the smell is strongest with your nose near the overflow rather than the drain opening, you have found it.
If the smell is stronger at the drain opening than at the overflow, the culprit is further down - our guide to kitchen sink smells and to why a sink gurgles covers those cases.
How to clean it without taking the drain apart
You do not need tools, and you do not need to undo anything under the sink. Pull the stopper out and set it aside. Fill the basin with warm water and add a cup of white vinegar - or a squirt of dish soap if you prefer. Let it sit for ten minutes. Then drain the basin while holding a cloth or a plug partly over the main drain opening, so the water is forced to leave through the overflow channel instead of straight down. Repeat once. For the hole itself, a small bottle brush, a pipe cleaner, or even a folded corner of a cloth worked into the opening will lift out the film. Finish by pouring a little vinegar directly into the hole with a funnel.
If the overflow is blocked
A blocked overflow gives you two symptoms. The obvious one is that the safety function stops working - fill the sink with the stopper closed and the water level can reach the rim. The subtler one is drainage: because the overflow is also an air path, blocking it makes the sink drain more slowly and can make it gurgle. Clear it with a pipe cleaner or a short length of wire worked gently through the hole and down the channel, then follow with the vinegar soak. Never force anything rigid far into the channel - the walls are thin.
| What you notice | What it means | What to do |
|---|---|---|
| Musty smell near the basin | Biofilm in the overflow channel | Vinegar soak, then a bottle brush in the hole |
| Water almost reaches the rim | Overflow channel partly blocked | Clear it with a pipe cleaner or soft wire |
| Sink drains sluggishly | Overflow blocked plus build-up in the drain body | Clear the overflow, then the stopper and trap arm |
| Gurgling after draining | Air cannot get in through the overflow or vent | Clean the overflow, then check the vent |
| Water on the floor, drain closed | Overflow blocked while the tap was left running | Clear the overflow immediately |
The overflow and a slow drain are connected
The overflow channel joins the drain body above the trap, so build-up in one affects the other. If your sink is slow and it also smells, the two jobs are usually the same afternoon's work: clean the overflow channel, pull the stopper and clear the hair, then flush with hot water. Doing only one of them is why the smell or the slowness keeps coming back.
Five minutes, twice a year
Twice a year is enough: once in spring and once in autumn, put it on the same list as replacing the vacuum filter or cleaning the range hood. Vinegar soak, bottle brush in the hole, hot water flush. Five minutes, no disassembly, and you have removed the most common source of sink odour without buying a product for it.
The bottom line
The hole near the top of your sink is an overflow and an air vent, not decoration. It keeps water off your floor when a tap is left running, and because it is damp and never flushed, it is the part of the sink that smells worst. Clean it with vinegar and a brush twice a year, never cover it up, and your sink will both drain better and stop smelling like a drain.
Frequently asked questions
What is the hole in the sink for?
It is an overflow. If the tap is left running with the drain closed, water escapes through it into the drain body instead of over the rim onto the floor. It also lets air into the drain so water flows more smoothly.
Why does my sink overflow smell?
Because it is a damp, dark channel that almost never gets flushed. Soap scum and biofilm build up inside it, and that is the smell most people blame on the drain itself.
How do I clean a sink overflow hole?
Pull the stopper, fill the basin with warm water and a cup of vinegar, let it sit ten minutes, then drain it while partly blocking the main drain so water is forced through the overflow. A small bottle brush finishes it off.
Can I cover the overflow hole?
You can, but you should not. A cover defeats the safety function and traps moisture in the channel, which makes the smell worse.
Related reading
- Why does my sink gurgle? (And how to stop it)
- Sink stopper won't hold water? 5 fixes for a leaking pop-up drain
- Why does my bathroom sink drain slowly? (And what actually fixes it)
If the overflow is fine but the stopper is not, the one we make is our pop-up sink drain stopper with a hair catcher.