The coriolis effect toilet myth is a staple of airport conversation: water supposedly drains counterclockwise north of the equator and clockwise south of it. It does not. In any normal bathroom, the direction is decided by the shape of the bowl, the angle of the jets, and whatever currents were left in the water when you pulled the plug.
The underlying physics is real. The mistake is scale.
What the Coriolis Effect Actually Is
Earth rotates. A point on the equator moves east at about 1,670 km/h; a point at 45 degrees latitude moves at roughly 1,180 km/h. Air moving north from the equator carries its extra eastward speed with it, so it curves east relative to the ground below.
In a rotating frame, that shows up as an acceleration: 2Ωv sin φ, where Ω is Earth’s rotation rate, v is the speed of the moving object, and φ is latitude. It deflects motion to the right in the Northern Hemisphere and to the left in the Southern.
How Weak It Is
At 45 degrees latitude, water moving at 1 meter per second feels a Coriolis acceleration of about 0.0001 m/s². Gravity is 9.8 m/s². The ratio is roughly one part in a hundred thousand.
Now consider what else is acting on the water in a toilet: a jet entering at an angle, an asymmetric rim, a drain offset from center, and the residual swirl from the last flush. Those effects are thousands of times stronger.
The Two Experiments Everyone Cites
The myth persists partly because the effect has been demonstrated — under conditions no bathroom could reproduce.
In 1962, Ascher Shapiro at MIT filled a large circular tank, let it sit for about 24 hours to settle, and drained it through a small hole at the center. The water developed a counterclockwise rotation, consistent with the Northern Hemisphere. In 1965, Lloyd Trefethen at the University of Sydney ran the southern version and reported the opposite direction in Nature.
Both experiments required isolating the tank from vibration, keeping temperature uniform, and waiting long enough for residual motion to decay. A bathtub in a house with a refrigerator compressor does not qualify.
Where the Effect Genuinely Dominates
The Coriolis term grows with the size and duration of the motion. That is why it governs weather systems and almost nothing in a kitchen.
| Scale | Example | Does Coriolis matter? |
|---|---|---|
| Centimeters, seconds | Toilet, sink, coffee cup | No — overwhelmed by geometry |
| Hundreds of meters | Small pond, harbor eddy | Rarely, and only if undisturbed |
| Hundreds of kilometers | Hurricanes, ocean gyres | Yes — it sets the rotation |
| Thousands of kilometers | Jet stream, global circulation | Yes — a defining term |
Hurricanes are the clean example. They spin counterclockwise in the Northern Hemisphere and clockwise in the Southern, and they cannot form within about 5 degrees of the equator, where the Coriolis term vanishes.
Why the Bathtub Test Fails
Water carries vorticity from every disturbance it has experienced. Filling the tub, stirring with your hand, even the shape of the faucet aerator leaves rotation that persists for hours. The Coriolis term is too small to erase it.
To make the experiment work you would need a perfectly symmetric basin, absolutely still water, uniform temperature, and days of patience. Even then, the result is fragile — which is why replication attempts have produced mixed outcomes and why the 1960s papers were notable in the first place.
Why People Believe It Anyway
The claim has three features that make it stick: it is counterintuitive, it is teachable in one sentence, and it flatters the speaker with inside knowledge. It also borrows credibility from a real phenomenon, which is the most effective kind of borrowing.
Tourist demonstrations in Kenya and Ecuador exploit the gap. A basin is carried a few meters across the equator line, water is poured by hand, and the showman’s pour direction does the rest.
The Honest Version
The coriolis effect toilet claim is wrong for a reason worth remembering: a real force can be irrelevant at small scales. Physics does not scale linearly with intuition, and the contest between forces is usually settled by orders of magnitude, not by which effect sounds more exotic.
Your sink drains the way it does because of how it was built. The planet is not involved.
Keep Exploring
- How do ocean currents work? — where Coriolis shapes the flow
- Weather vs climate — the scale where the effect dominates
- What is the water cycle? — the rest of the story for that water
