1. How the hole near the rim stops water from overflowing when the drain is plugged
Imagine closing the plug of a washbasin and letting water fill up. As the water level rises and reaches the height of the hole near the rim, the water flows into that hole. The hole helps prevent the water from crossing the edge and spilling onto the floor. This feature is called an "overflow." Several explanations state that it is added to prevent overflowing when water is stored with the plug closed [1][3].
This works on the same principle as making a hole slightly below the rim of a bucket; it makes it harder for water to spill over the top. However, in a washbasin, the hole does not dump water outside. Instead, it sends the water into the drainage pipe. So, where exactly does the water that enters the hole connect to the drainage system?
2. Where the water from the hole goes: it joins the drain pipe before the U-shaped trap
The water that enters the hole flows through a pipe behind the washbasin. It joins the main drain pipe from the basin bowl just before the U-shaped pipe [1]. This U-shaped pipe, known as a trap, always holds some water. This water acts like a lid, preventing bad smells and insects from the sewer from coming up [1].
There is a reason for joining the overflow water on the near side of the trap. According to an explanation of US plumbing codes (UPC), if an overflow is added to a sink or basin, its pipe must connect only to the fixture side (before the trap). The explanation says it must not bypass the trap and connect directly to the sewer side [4]. This is a rule from American regulations, not an explanation of Japanese law.
This means the overflow water also follows a path that goes through the water seal before heading to the sewer. The hole is designed so it does not break the system that stops smells. By the way, this hole is said to have another function besides letting water escape.
3. How the hole may also work as an air passage so water drains smoothly
If you turn a water bottle upside down, it may make a glugging sound and drain slowly. One reason may be that air has no path to enter, which makes it difficult for the water to flow. There is an explanation that even with the narrow drainage of a washbasin, if air cannot enter, the water may flow as if clogged, causing glugging sounds or backflow. It is also said that air entering through the hole makes the water flow more easily [6].
A professional magazine on residential construction in other countries notes that while this is not the original purpose, the overflow has a secondary benefit of letting air escape from the drain pipe as water flows down [5]. The writer of this magazine also writes that vessel-style sinks without holes tend to have slower drainage. However, this comes from explanatory articles and a trade magazine, not from technical plumbing documents or papers. Drainage speed also depends on pipe thickness, slope, and other air paths, so it is not certain that water always flows faster. The primary purpose of the hole remains preventing overflow.
So, what happens with washbasins that do not have this important overflow hole?
4. Why some washbasins have no overflow hole at all
According to an explanation by an Italian sanitary ceramics manufacturer, traditional washbasin overflows often had three small holes in a row. Later, designs with a single round hole became more common, and recently, some washbasins with more design-focused shapes have no holes at all [2]. This is the explanation from one manufacturer, and this source does not say when these designs changed.
A washbasin can work without a hole because the hole is not required. An explanation of US plumbing codes (UPC) states that overflows are not required for washbasins, sinks, or bidets [4]. This article does not cover Japanese regulations.
In washbasins without holes, you must watch the water level yourself and turn off the tap when filling the basin. The hole is a component that creates a safety margin if you overfill, and whether to include it is a design choice.
5. How to compare the holes in washbasins at home and at school
Look at the washbasin at home. Check if there is a hole near the rim. The number and position of holes may differ from one washbasin to another. If you compare it with washbasins at school or in public places, you might find one without a hole. When testing water storage, do not leave the sink area, and observe together with an adult.
The hole is small and prone to collecting soap residue, water stains, mold, and slime due to moisture [1]. Because it is hard to fit a brush inside, it is generally explained that cleaning with foaming detergents is common.
To find more information, look for materials from the sewerage department of your local municipality. They may explain drainage equipment and traps. Try searching for terms like "trap" or "water seal." We have learned that the hole you look at every day is an escape route for excess water and connects to the drain just before the water seal that stops smells. If you find a hole of a different shape, imagining where the water goes can be fun.