Bathroom sink drains have a tendency to become slow, meaning that they become partially clogged and water can't empty out of the sink as quickly as it would with a clear drain. Most of the time, this is due to something actually in the drain, such as gunky buildup from soap, dirt, and hair. Yet there are a few other reasons that the sink may be draining slowly that aren't due to just a partial clog.

Debris Around the Drain

Soap, dirt, and even food particles, if you're rinsing off the occasional dish in that sink, can build up around the drain, between the metal ring and the sink surface. That creates a bit of an obstacle to water flow. It may look like just a film on the sink's surface, but it can narrow the path into the drain.

When you clean your sink, take some baking soda and sprinkle it around the drain and over the sink's surface. Take a damp paper towel and scrub the sink. The resulting baking soda paste won't hurt the sink's surface, but it will scrape up that residue, creating a smoother sink surface and better water flow. It will also make the sink a lot cleaner by removing discoloration due to that residue, as a bonus.

Stopper Height and Soap Buildup

Soap particles can get caught on the sides of the drain pipe and eventually build up to form a clog. But the underside of the stopper in the drain is also subject to this buildup, and it's not as easy to clean that off without disassembling the stopper so you can lift it out. The buildup under the stopper narrows the space between the stopper and the drain when the stopper is up, creating a backup when you try to empty the sink. The result looks like something is wrong with the drain, when it's not actually the drain itself.

Another cause could simply be that the stopper isn't all the way up. That stopper has to be pushed all the way up for the sink drain to be fully open. If the stopper is lower than normal, then the water flow down the drain will be slower than usual.

Try cleaning the sink's surface and the area under the stopper, and make sure the stopper is all the way up. If you're still having trouble with a slow drain, call a plumber or drain cleaning company to have the drain repaired.