healthHealth and Medicine

The "Law Of Urination" And Why You Should Go By The "21 Second Rule"

James Felton

James Felton

James Felton

James Felton

Senior Staff Writer

James is a published author with four pop-history and science books to his name. He specializes in history, strange science, and anything out of the ordinary.

Senior Staff Writer

Don't hold your pee in until you physically have to grab your penis.

Don't hold your pee in until you physically have to grab your penis. Image credit: Polkadot_photo/

In 2015, an Ig Nobel Prize for physics was awarded for taking high-speed footage of animals peeing in zoos, and gawping at more videos of animals urinating via YouTube. The team modeled the fluid dynamics involved in peeing for a variety of different sized animals, and found what they termed the "Law of Urination". 

It's a simple law, but a curious one: animals that are over 3 kilograms (6.6 pounds) empty their bladders over about 21 seconds. In smaller animals – for example, rats – urination can happen in a fraction of a second. The research helped confirm that pee duration was not just to do with bladder pressure, but also related to our good friend gravity. In smaller animals, pee time is constrained by the surface tension of urine.


"How can bladders of both 0.5 kg and 100 kg be emptied in nearly the same duration? Larger animals have longer urethras, and so greater gravitational force driving flow. These long urethras increase the flow rate of larger animals, enabling them to perform the feat of emptying their substantial bladders over approximately the same duration," the team wrote in their paper.

"In this study, we find the urethra is analogous to Pascal’s Barrel, acting as an energy input device. By providing a water-tight pipe to direct urine downward, the urethra increases the gravitational force acting on urine and so the rate that urine is expelled from the body. Thus, the urethra is critical to the bladder’s ability to empty quickly as the system is scaled up."

Here's a recap of Pascal's Barrel, in which a small amount of water causes a large jug to burst because of that added energy.

While it's fun just to know how animals pee and for how long it's also useful to know for diagnosis of bladder conditions and use of mouse and pig models for urological studies. Using the data they collected, they were able to produce an equation that should tell you how long each animal should pee for. Deviation from this urination time could be an indication of poor bladder health, swelling, infection, or prostate problems.

The 21-second rule

Urologist Nicole Eisenbrown told Well and Good that this "21-second rule" can be used to help keep an eye on your own health and habits. Basically, if you time yourself peeing (over a period of time, not just one sitting) and find that you take significantly longer or shorter than 21 seconds, it can indicate that you are holding it in for too long, or not enough.

Say you are in a profession where it's difficult to get away to the bathroom (e.g. a teacher) and thus hold it in for as long as you can. This can enlarge your bladder by chronically over-extending it, and could cause you functional problems down the line. 

Should you pee too often (and peeing for much less than 21 seconds would be a good indicator of this), you may end up with an "overactive bladder": your bladder feels full and you need to pee, even when you shouldn't. 

As mentioned in the study above, altered flow rates can be an indication of more serious health problems, as well as other smaller ones. If you find yourself peeing frequently, and so the volume is smaller, it can be a sign of bladder stones, for example, or cystitis. Longer times spent urinating at lower pressure could indicate prostate problems


It should be noted that as males get older, flow rates do go down, but should you have any concerns about your urination – particularly if you are experiencing other symptoms such as pain or discomfort – it is worth discussing with a physician.


healthHealth and Medicine
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  • urination