"A misconception in popular culture is that the Coriolis effect determines the direction in which bathtubs or toilets drain, such that water always drains in one direction in the Northern Hemisphere, and in the other direction in the Southern Hemisphere. This urban legend has been perpetuated by several television programs, including an episode of The Simpsons and The X-Files[1]. In addition, several science broadcasts and publications (including at least one college-level physics textbook) have made this incorrect statement.[2]. Many people who misunderstand the Coriolis effect compound their misunderstanding by claiming that drain water spins clockwise north of the equator and counterclockwise south of it, which is reversed from direction of spin that would result from the Coriolis force if it were a determining factor.
The Coriolis effect is a few orders of magnitude smaller than various random influences on drain direction, such as the geometry of the sink, toilet, or tub, and the direction in which water was initially added to it. For example, consider a bathtub where draining creates a waterlevel difference of 3 cm over 60 cm, giving a pressure gradient of 500 N/m3. Now assume the water is draining at a speed of 50 cm/s. At a latitude of 45 degrees, this would give rise to a Coriolis acceleration of 0.05 N/m3, or only 0.01 % of the pressure gradient.
Most toilets flush in only one direction, because the toilet water flows into the bowl at an angle[1]. If water shot into the basin from the opposite direction, the water would spin in the opposite direction[2].
The Earth rotates once per day but a bathtub takes only minutes (and a toilet only seconds) to drain. When the water is being drawn towards the drain, the radius with which it is spinning around it decreases, so its rate of rotation increases from the low background level to a noticeable spin in order to conserve its angular momentum (the same effect as ice skaters bringing their arms in to cause them to spin faster). As shown by Ascher Shapiro in a 1961 educational video (Vorticity, Part 1) [3], this effect can indeed reveal the influence of the Coriolis force on drain direction, but only under carefully controlled laboratory conditions."