It is one of the first things some overseas visitors want to check when they land in Australia: does the toilet really flush the other way down here? The theory goes that because of the Earth’s spin, water swirls clockwise down the drain in the Southern Hemisphere and anticlockwise in the Northern Hemisphere. There is real physics behind the idea, but in your bathroom, it makes no practical difference at all. And as it turns out, most Australian toilets do not swirl much in the first place.
Blame Bart Simpson
If you grew up in the 1990s, there is a good chance you first heard about this from The Simpsons. In the 1995 episode “Bart vs. Australia”, Lisa explains that water drains in the opposite direction in the Southern Hemisphere, so Bart makes a reverse-charge phone call to Australia to check, sparking a diplomatic incident. Later, the family visits the American embassy, where a gadget has been fitted to make the toilet swirl the “correct” American way. It was a great joke, and it helped lodge the idea in millions of heads. Michael Palin’s 1992 BBC travel series Pole to Pole helped too, with a scene at the equator in Kenya where a local man appears to show water draining in different directions on either side of the line, as Mental Floss notes.
The real force behind the myth
The myth is built on something real: the Coriolis effect, named after the French scientist Gaspard-Gustave de Coriolis, who described it in 1835. Because the Earth is spinning, anything that travels a long way across its surface, such as air in a weather system, is deflected slightly. In the Northern Hemisphere, the deflection is to the right; in the Southern Hemisphere, to the left. Over hundreds of kilometres, that small nudge adds up, which is why big storm systems spin in opposite directions in each hemisphere. Cyclones off Queensland and Western Australia turn clockwise, while hurricanes in the Atlantic turn anticlockwise. So it seems natural to assume water going down a drain should follow the same rule.
Why it does not work in your bathroom
The problem is scale. A cyclone can be hundreds of kilometres across and last for days. A toilet bowl or basin is about 30 cm wide and empties in seconds. At that size, the Coriolis force on the water is about 10 million times weaker than the pull of gravity, as Dr Karl Kruszelnicki of the University of Sydney explains in Australian Geographic. That is so tiny that it is completely swamped by everyday factors: the shape of the bowl, where the drain sits, the direction the water came in from, small temperature differences, and even air moving across the surface. Any of those can set the water spinning one way or the other, and the Earth’s rotation has no say in the matter.
In a toilet, the answer is even simpler. Which way the water swirls, if it swirls at all, depends on how the bowl was designed and which way the flush jets are angled, as Live Science explains. Two identical toilets, one in Sydney and one in Seattle, will swirl the same way.
The experiments that proved it can be done
Here is the twist that makes this more than a simple myth. Under perfect laboratory conditions, the Coriolis effect really can decide which way water drains. In 1962, Professor Ascher Shapiro, a fluid mechanics expert at the Massachusetts Institute of Technology, filled a round, shallow tank about 2 metres across and 15 cm deep, let the water settle until it was completely still, then drained it very slowly through a small hole in the middle. The water took about 20 minutes to drain, and for the first 12 to 15 minutes, there was no visible spin at all, according to Dr Karl. Only near the end did a vortex form, and when every precaution was taken, it turned anticlockwise every time.
A few years later, Dr Lloyd Trefethen and his colleagues at the University of Sydney repeated the experiment with very similar equipment and consistently got clockwise rotation, exactly as the theory predicted for the Southern Hemisphere. So, in one sense, Australian water really does have its own spin. It is just far too faint to show up anywhere except in a large, perfectly still tank drained with enormous patience.
The equator trick, explained
If you have travelled to the equator in Kenya, Uganda or Ecuador, you may have seen a guide perform the famous demonstration: a bowl of water drains clockwise on one side of a painted line, anticlockwise on the other, and straight down when it is right on the line. It is fun to watch, but it is a trick. The guide controls the result by how the water is poured into the bowl: still water drains with no swirl, while water poured in with a little momentum spins whichever way it was poured, as a G Adventures guide explains. Ironically, the Coriolis effect is actually weakest at the equator, so if anything, it should make the least difference there. And as some travellers have pointed out, the painted line at popular sites is not always on the exact equator anyway.
The Australian twist: our toilets barely swirl
Here is something many visitors notice once they actually look. Many toilets around the world, including many Australian ones, do not produce a big swirling vortex at all. They use a wash-down design, where water rushes in from around the rim and pushes everything straight down the outlet, as The Points Guy notes. The dramatic whirlpool many people picture comes from the siphon-style toilets common in North America, which use much more water. So if you go looking for a clockwise swirl in an Australian loo, you may be disappointed for a reason that has nothing to do with physics.
Australia also changed toilets for the whole world. In 1980, Bruce Thompson, a designer at the bathroom company Caroma in Adelaide, developed the first two-button dual-flush cistern, giving users a choice between a full and a half flush, as Architecture and Design explains. In a trial in a small South Australian town, it saved each household around 32,000 litres of water a year. Dual flush is now standard in Australia and has been exported around the world. So the next time an overseas visitor asks you about Australian toilets, the dual-flush button is a far better story than the swirl.
Try it yourself
If you want to test the Coriolis effect at home, a toilet is useless, but a bathtub or a large, shallow container gives you a sporting chance. Fill it, then leave it completely alone for as long as you can, ideally a full day, so that any swirling from filling it up has died away. Keep the room still, with no fans or draughts. Then pull the plug as gently as possible, without stirring the water, and drop a few small bits of paper on the surface to show which way it turns. Try it several times. You will almost certainly get different results on different tries, which is exactly the point: everyday disturbances win out over the Earth’s spin. To get a reliable answer, you would need Shapiro’s patience and a much bigger, flatter tank.
The verdict
So, does the toilet really flush the other way in Australia? No. The Coriolis effect is real, it shapes our cyclones, and in a carefully controlled laboratory tank in Sydney, it really did make water drain clockwise. But in a toilet or sink, it is millions of times too weak to matter, and the direction of the swirl comes down to the design of the bowl. The best answer to give a visitor is that Australian toilets are not upside down, just more efficient. If you enjoy checking Southern Hemisphere myths, my post on red sky at night, sailors’ delight tests another, and why is the sky blue? looks at more everyday physics.
Sources
- Australian Geographic: Dr Karl, which way does water go down a drain?
- Live Science: Which direction does toilet water swirl at the equator?
- The Naked Scientists: Can you detect the Coriolis effect in your sink? (Dr Karl interview)
- Mental Floss: Australian toilets don’t flush backwards because of the Coriolis effect
- G Adventures: Busted! Top 3 equator line tricks debunked
- The Points Guy: Do toilets flush backwards in the Southern Hemisphere?
- Architecture and Design: Four water-saving toilet innovations