Changing Planet

New Law of Urination: Mammals Take 20 Seconds to Pee

Call it the other Golden Rule: Scientists have found that all mammals weighing more than 2.2 pounds (a kilogram) empty their full bladders in about 20 seconds.

Like many new parents, David Hu, a mechanical engineer at the Georgia Institute of Technology, has changed a lot of diapers. Unlike many new parents, however, these soggy diapers caused Hu to think about the physics of urination.

“While I was changing these diapers, I was wondering how it would be different for different animals. How much fluid would they create and how long would it take to leave the body?” Hu said. (Also see “Growing Teeth and Four More Odd Uses for Urine.”)

“The physics of urination—what the forces are and how they affect how quickly urine comes out, is not totally understood, even though it’s a really old problem.”

Although it might sound silly on the surface, urination is actually serious business in the medical and veterinary worlds, especially during the aging process. Many men get enlarged prostates as they get older, which can narrow the urethra and impede urine flow. Veterinarians have been looking for a quick and easy way to identify problems with animals’ urinary tract.

Pee Cam

First, Hu wanted to know how urination varied from species to species. The bladder of a large domestic dog can hold 1.4 liters (about 0.4 of a gallon) of fluid, or roughly the amount of a large bottle of soda. An elephant’s bladder, however, can hold 160 liters of fluid, or enough to fill three large garbage cans. Hu wanted to know how this size difference affects the urinary tract and urine flow.

Enter the pee cam. Three of Hu’s graduate students at Georgia Tech used high-speed cameras to record peeing animals at Zoo Atlanta and elsewhere.

They also measured how much pee was produced by each of these animals, which ranged from rats to jaguars to elephants. The scientists supplemented their research with YouTube videos from zoo visitors. Last, the researchers obtained measurements of the animals’ bladder and urethra widths and lengths from other researchers.

Hu and colleagues suspected that bigger animals would take longer pee breaks than smaller animals, since they had to expel larger volumes of urine. But when they began determining the urination duration of each of these animal species, that wasn’t what they found.

“Even though you have thousands of times more urine, it’s coming out in the same amount of time, which is around 20 seconds,” said Hu, whose study appeared recently on the Cornell University website arXiv. (Also see “Urine Battery Turns Pee Into Power.”)

Urination Law Explained

Urinary tract measurements helped to solve the mystery of how animals of such different sizes all pee for the same amount of time.

Larger animals not only had larger bladders, they also had longer and wider urethras. The length of the urethra increases the force of gravity on the urine, which in turn increases how fast pee flows out of the body.

A wider urethra also increases flow rate by increasing the volume of urine that can leave the body at the same time. These increases correlate with the increase in body mass, such that an elephant can empty its bladder in the same amount of time as a cat. (Also see “Turtles Urinate Via Their Mouths—A First.”)

Saying an elephant pees for the same amount of time as a domestic cat seems impressive enough, but it gets even more impressive when you consider just how much urine the elephant produces: several bathtubs’ worth. It’s like having 60 showerheads all going at full blast, Hu said.

For small mammals—those weighing less than 2.2 pounds (a kilogram)—Hu’s Universal Law of Urination wasn’t as accurate. Their urethras were so narrow and short that the surface tension of the urine slowed flow down to mere droplets. Their bladders were also much smaller, so a few drops could successfully empty the bladder. These animals, like rats and mice, can successfully urinate in less than a second.

Animals benefit from such relatively short pee breaks because such pit stops increase risk of predation, Hu noted. Faster urine flow may also help prevent urinary tract infections by flushing out the system.

Now I just have to fight the urge to bring a stopwatch with me into the bathroom.

Follow Carrie Arnold on Twitter and Google+.


Carrie is a freelance science writer living in Virginia. When she's not writing about cool critters, she's spending time outside, drinking coffee, or knitting. You can visit her website at
  • Dennis

    Could this be used in some way to help people figure out when they really need to pee physically rather than mentally feel the need to expel urine?

  • am-sino

    Guess he didn’t time a camel’s pee?

  • Gotham_Greg

    Something isn’t quite right in the graphic — the available literature reports that an elephant’s bladder has a capacity of no more than 18 liters — 42 gallons is more like the total daily output of elephant urine.

  • mrbeverage

    so the “Rule of 20” applies to all mammals weighing over 2.2 pounds. hmmm, I wonder if that is true for both sexes, as i have noticed that bathroom lines in bars move in disproportionate speeds when divided between men and women… although i am sure there are other variables i am not considering. and this study also neuters the expression, “I had to pee like a Russian race horse”… as the mentioned mammal expels his pee the same time as a Russian Chihuahua.

  • Cam

    I agree with Greg from New York. There is no way an elephant will willingly save up 160 L or “three large garbage cans” worth of urine. That’s 160 kg (353 lbs) of pee. The literature I have read suggests that elephants urinate often, since their bladder is small relative to their total body size, in comparison to other mammals.
    Also peeing at a rate of 2 gallons a second would be extraordinary indeed. Many household well water systems deliver around 2 to 7 gallons per minute. Just try and fill a gallon milk jug with a hose and see how long it takes. An elephant would literally need firehose-like abilities to deliver 2 gallons a second.

  • Cam

    I might also mention that a large garbage can, around here, typically has a 110 or 120 L capacity, although if you fill it to the top, the sanitation workers may legally refuse to lift it.

  • Chrisj

    “The length of the urethra increases the force of gravity on the urine”

    No, it really very definitely doesn’t. How close to vertical the urethra is might have some affect on the acceleration the urine receives (though I’d expect the effect to be small relative to the squeezing of the bladder muscles), but the force of gravity remains constant.

  • Sadhana

    Wow, I am sure this information has helped society immensely !!
    Still I appreciate the efforts, wish I had a job like that of the scientists in this study !!

    Kudos 2 u guys, keep discovering more weird amazing facts !!

  • Ben

    Someone’s lost a decimal point and added a zero to the capacity of an elephant’s bladder… 1.4 litres for a domestic dog, and more like 16.0 litres for an elephant, not 160 litres.

  • Nabil

    @Cam I have worked and studied elephants in the wild and it is not unconceivable that they can drain 2gal/secs – see these I took while in the field:

  • JoBou

    Note that the chart states the bladder volume and not the pee volume. If we take humans as exemple, bladder can contain around 500 to 600 mL but the urge to pee is sensed at around 150 mL (about 20% of full capacity). We can speculate that this is the same for elephants, which would reduce the flow rate to about 1.6 L/s (0.4 galon/s) down from 2 galon/s.

  • Pandahead

    MR.Beverage as a female that gets often frustrated with long bathroom lines I can tell you that it is because most girls and women can’t just sit down and pee and get out of the cubicle. You will sometimes here them flush the the toilet even and not come out for another couple of minutes because they’re are messing around doing who knows what.

  • Paul Ochiri

    That this applies to humans at all times of the day is pretty hard to believe. I have to start using a stop watch.

  • Sofia Talvik

    I agree with Gotham_Greg. Although the elephants are large it seems unlikely their bladder would be that big, and my research online also came up with 18 liters as apposed to 160 liters that this article states. Quite a difference!

  • Robert D. Childers M.D.

    I’m suprised a M.E. doesn’t remember fluid dynamics. Resistance to flow increases by the first power of the length–a longer tube has slower flow than a shorter tube.
    Flow varies by the 4th power of the radius. Double the diameter and you increase flow 16 fold. We do use flow rate in humans to evaluate for obstruction, and the rates fit these findings.

  • Dave LaRonde

    I wonder what it was like for a researcher to follow an animal around, wait until the time is right, and hit the stopwatch. Not to mention doing it again and again to get an average.

    “What do you do for a living?”
    “I measure the time it takes for an elk to take a whiz.”
    Can there be any more meaningful avocation?

  • Cam

    I had a look at some bison bladders (they are available from several internet vendors) and I strongly disagree that they could contain the volume suggested in the graphic. More like 5.5 litres, maybe.

    I suggest that the maximum flow rate has been multiplied by the average urination time, to arrive at a erroneous total expelled volume value.
    Maybe an elephant can pee at two gallons per second maximum, but not from start to finish.

  • katie

    Why are you are all so interested in pee? Does it matter?

  • stevie

    misleading headline unless this is a casual blog not to be taken seriously. there would be no variation in urination times if this was a universal law of the universe which you seem to grant this particular study.

  • Walter Pérez.

    Interesting! But I would like to suggest to mention about urine therapy in humans. It is an excellent way to avoid quemotherapy effects in our body.

  • Afro-Black

    Katie, why yes it does… it really, really does.

About the Blog

Researchers, conservationists, and others share stories, insights and ideas about Our Changing Planet, Wildlife & Wild Spaces, and The Human Journey. More than 50,000 comments have been added to 10,000 posts. Explore the list alongside to dive deeper into some of the most popular categories of the National Geographic Society’s conversation platform Voices.

Opinions are those of the blogger and/or the blogger’s organization, and not necessarily those of the National Geographic Society. Posters of blogs and comments are required to observe National Geographic’s community rules and other terms of service.

Voices director: David Braun (

Social Media