That little hole is the sink overflow. A channel inside the bowl carries water from it down into the drain, above the trap, so a basin with the stopper closed can't spill over the rim. It also lets air into the drain so a full sink empties faster. Most bathroom sinks have one; kitchen and vessel sinks often don't.
How the sink overflow works
The overflow has two jobs: catch rising water and let air in. Both happen because the hole connects to the drain before the trap.
### It stops a spill when the stopper is closed
Picture a sink with the pop-up stopper shut and the faucet left on. The water climbs until it reaches the overflow hole, then pours through the hidden channel into the drain body under the bowl. Moen explains that an overflow drain has a hole in the upper drain assembly that lines up with a channel in an overflow-style sink, and says this prevents sinks from overflowing when the drain is closed or clogged.
The plumbing code sets how that connection has to work. The 2024 International Plumbing Code (IPC), Section 405.8, says that where a fixture has an overflow, "standing water in the fixture will not rise in the overflow when the stopper is closed, and water will not remain in the overflow when the fixture is empty." Section 405.8.1 adds that the overflow "shall discharge into the drainage system on the inlet or fixture side of the trap."
That second rule matters in practice. The overflow ties in above the P-trap. So if the clog is in the trap or the pipe past it, the overflow has nowhere to send the water either. It protects you from a closed stopper, not from a fully blocked drain. It also has limits on flow. Signature Hardware notes that overflows "aren't made to handle rapidly flowing water," so a faucet running full blast can still beat it.
### It lets air into a draining sink
Water leaving a full basin has to trade places with air in the drain. With only one opening, the water and air fight each other at the drain hole, and the sink glugs slowly. Fixture maker Signature Hardware puts it this way: "The overflow allows air into the piping so your sink can remove water quickly." That's why a full sink often gurgles from the overflow hole as it empties. Air is moving through it, which is normal.
### Where you'll find one
| Fixture | Overflow? | What the code says |
|---|---|---|
| Bathroom sink (lavatory) | Usually, but not always | No overflow required; if there is one, IPC 405.8 applies. The waste outlet must be at least 1 1/4 inches with a strainer, pop-up stopper or crossbar (IPC 419.3). |
| Bathtub | Usually | Where installed, the overflow connects upstream of the trap and must drain whether the stopper is open or closed (IPC 407.2). |
| Kitchen sink | Rarely | No overflow required. |
| Vessel sink | Often not | Sold both ways; check the sink's spec sheet. |
| Toilet tank | Always | The overflow tube is required and sends extra water into the bowl (IPC 415.3.2). |
So the code doesn't make a bathroom sink have an overflow. It sets the rules for one that's there.
Why the overflow smells or gurgles, and how to clean it
A smelly overflow is almost always gunk, not sewer gas. The channel inside the bowl gets splashed with soap, toothpaste, hair and skin oils every day, and nobody ever scrubs it. That film holds moisture and grows the slime that gives off a musty or rotten smell. Our page on black slime in drains covers why that film forms.
A true sewer smell is a different problem. If the smell is strong and comes from the drain itself, the P-trap may have dried out, and our guide to why a bathroom drain smells like sewer walks through that fix first.
### How to clean a sink overflow
- 1Pull the stopper. Most pop-up stoppers lift out or unscrew. Clean it while you're there.
- 2Brush the channel. Push a thin bottle brush or flexible pipe brush into the overflow hole and work it up, down and sideways. You'll likely pull out gray or black gunk.
- 3Flush it. Put a small funnel in the overflow hole and pour a few cups of hot tap water through it. Watch for the water to come out at the drain.
- 4Treat what's left. An enzyme drain cleaner poured through the funnel and left overnight breaks down the organic film. Don't mix cleaners, and never combine bleach with anything else.
- 5Flush again with hot water in the morning.
Repeat every few months in a sink that gets heavy use. That's it.
### When gurgling is a problem
A little gurgle from the overflow while a full basin drains is air doing its job. Gurgling while the sink drains slowly, or a sink that gurgles when a toilet flushes, points somewhere else: a partial clog or a vent problem. See our pages on fixing a slow-draining sink and what a plumbing vent does. When you plunge a sink, cover the overflow hole with a wet rag first, or the pressure just blows out through the overflow.
Sinks without an overflow
Plenty of good sinks skip it. Vessel bowls that sit on top of the counter are often made without an overflow, and kitchen sinks rarely have one. The upside is a cleaner look and one less channel to collect gunk. The downside is simple: close the drain, walk away, and the sink can spill.
### Match the drain to the sink
This is where people get caught. Drains are sold with overflow and without overflow, and they aren't interchangeable.
- An overflow drain has a slot or hole in its upper body. It only works if the sink has an overflow channel for that slot to line up with.
- A non-overflow drain has a solid body. It's meant for sinks with no overflow channel.
Moen warns that if an incompatible style of drain is installed, water can leak under the sink, and it doesn't recommend modifying either type to fit. Put an overflow drain in a sink without an overflow and water can come out of that slot below the bowl, into the cabinet. Put a non-overflow drain in a sink that has an overflow and the overflow channel has no way to empty into the drain.
### Pop-up or grid drain?
On a sink without an overflow, you still get to pick how the drain behaves:
| Drain type | How it works | Good for |
|---|---|---|
| Push-button or pop-up (non-overflow) | Press to close, press to open; holds water | Vessel sinks where you want to fill the bowl |
| Grid drain (non-overflow) | Open grate, no stopper, can't be closed | Vessel sinks where you never fill the bowl |
| Overflow pop-up | Slotted body lines up with the sink's overflow channel | Standard bathroom sinks with an overflow hole |
Sink maker LACAVA says its grid drain "does not close and will allow water to drain out at all times," and recommends grid drains for very shallow sinks or sinks without an overflow. That makes a grid drain the low-risk pick for a kid's bathroom with a vessel sink. LACAVA also notes that its lift-rod pop-up drain can only be used with sinks that have an overflow, a good reminder to check the drain's spec before you buy.
Should you worry about yours?
Most of the time, no. Clean the overflow when it smells, keep the stopper open when you're not using the water, and make sure any replacement drain matches the sink. Call a plumber when water shows up in the cabinet after a drain swap, when the sink drains slowly even after cleaning, or when the smell comes back fast. Those point to a mismatched drain, a clog past the trap, or a venting problem.
HQ Plumbing and Air is licensed and insured, offers free estimates with upfront pricing, and can clear a slow sink drain or install the right drain for your sink.