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21 Fascinating Facts Behind the Mystery of Pi

Find out why this ancient mathematical constant gets an annual celebration—one that's growing every year!

PI symbols and circle formulas drawn on a chalkboard
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The mystery of pi

Pi is a mathematical mystery that has captivated people for thousands of years. There’s even a holiday dedicated to this mystery—Pi Day, which falls on 3/14. People all over the world celebrate Pi Day in numbers ways, from cracking pi jokes and cashing in on Pi Day deals to learning more about the never-ending number. We couldn’t believe some of these fascinating facts about pi—how many do you know? Read on to unlock parts of the mystery.

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People have been using pi for thousands of years

Pi (the Greek letter π, pronounced like the word “pie”) is the ratio of the circumference of any circle to the diameter of that circle, explains math instructor Steven Bogart in Scientific American. It equals roughly 3.14. No matter what size a circle is, the circumference will always be 3.14 times bigger than the diameter. Over 4,000 years ago, the ancient Egyptians and Babylonians had figured out this constant and were using it to make calculations. In the 18th century, mathematicians gave the number the name “pi.” Don’t forget to check out these funny math puns that will make anyone laugh.

two Pi Pies, one displays the pi symbol itself and the other one shows the value of 3.14.
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We celebrate pi on March 14

Back in 1988, Larry Shaw of San Francisco’s Exploratorium science museum started observing March 14—get it? 3/14!—which also happens to be Albert Einstein’s birthday, as Pi Day. By 2009, the celebration had grown so big that Congress passed a resolution to make the designation official. The resolution states: “The House of Representatives supports the designation of a ‘Pi Day’ and its celebration around the world…and encourages schools and educators to observe the day with appropriate activities that teach students about Pi and engage them about the study of mathematics.” In another stamp of approval, in 2010, Pi Day got its own Google Doodle.

Pi Formula on Blackboard
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Pi is a never-ending number

One of the most well-known (but still mind-blowing) facts about pi that it is an irrational number. It can’t be expressed as a fraction; it doesn’t end with a repeating pattern (like the decimal expression of 1/3, 0.33333…, in which the threes repeat forever), or terminate after a certain number of decimal places (like 3/4, or .75). It just keeps going, going and going. So far, pi has been calculated to 100 trillion digits, thanks to Google Cloud. If you have math on the brain, try solving these math riddles—they’re trickier than you think!

Pi number written out on a chalkboard
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The digits of pi after the decimal point are random

Here’s a mind twisting pi fact: the trillions of digits of pi that have been calculated continue without any discernible pattern. Mathematicians have been looking for those patterns for centuries, but as far back as 1768, a self-taught Swiss-German mathematician and astronomer named Johann Lambert proved that pi is irrational. Something that is rational? Doing these math puzzles that test your smarts.

Pi Digits On A red Pencil
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More pi isn’t necessarily better

Interesting fact: While we know pi to trillions of places, we really don’t need them. Scientists can determine the spherical volume of the entire universe using just 39 places past the decimal, according to piday.org. NASA’s Jet Propulsion Laboratory only uses pi up to 15 decimal places for its robotic space and earth science missions. “For JPL’s highest accuracy calculations, which are for interplanetary navigation, we use 3.141592653589793,” explains engineer Marc Rayman. “There are no physically realistic calculations scientists ever perform” that would require more decimal points than that.

pi letter over a circle drawing near a pencil
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The name pi is much shorter than the number’s previous name

The number’s connection with the Greek letter π is actually pretty recent, considering that people have known about the quantity since the time of the ancient Babylonians. A British mathematician named William Jones was the first person to call the quantity π, in 1706. People theorize that he chose pi because it represents the Greek letter P, and pi can find the perimeter of a circle.

People have found math books from before 1706 that refer to the number as a lengthy Latin phrase that translates to “the quantity which, when the diameter is multiplied by it, yields the circumference.” Anyone who talks about math for a living, as well as anyone who likes finding excuses to eat pie, owes this Jones fellow one!

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State lawmakers once tried to round up pi to 3.2

If we don’t need all those decimal places in pi, wouldn’t it be easier to just call it 3.2? One of the more bizarre facts about pi is that In 1897, an Indiana doctor decided that the world should go ahead and use 3.2 for any calculations requiring pi. Dr. Edwin Goodwin proposed a bill in the state legislature. He even copyrighted this idea and planned to charge royalty fees for anyone who used it—except for those in the state of Indiana. After some debate, the state senate realized that the idea of using a law to change a mathematical constant was a silly one, and the law failed to pass.

Pi written out on Napkins
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Someone has memorized 100,000 digits of pi

According to The Guardian, Akira Haraguchi recited 100,000 digits of pi back in 2006. This impressive task reportedly took him a cool 16 hours and 30 minutes to complete. We have one word: Wow!

pi symbol
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You can borrow expert memorizers’ techniques

How in the world does someone memorize a string of 70,000 random numbers? Most record-holders (or just interested hobbyists) use an association technique. They bunch smaller groups of numbers together and memorize those: 14, then 15, then 92, then 65 and so on. Or they may look at each set of nine digits as a telephone number and memorize them that way.

Another strategy is to match each digit or small groups to a word, then make a story out of those words. Yet another method is spatial visualization, in which you picture a familiar place, then assign numbers to different spots in that place. To recall them, you mentally walk through the space and see the numbers as you go.

pi symbol made out of walking people
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Pi is a record-setter in more ways than one

Aside from Rajveer Meena’s achievement, other Guinness World Records have been awarded to pi-themed accomplishments. In 2014, 589 people at a grammar school in Germany formed the largest human pi symbol. And in 2017, 520 teachers and students in Todi, Italy, formed the longest human representation of pi digits. The city’s mayor held up a sign bearing the number three, and then each person after him stood in for a digit of pi after the decimal place.

geometric blocks in a row
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Pi has many real-world uses

It’s not all fun and games and feats of memory: Scientists use pi every day to make important calculations, such as determining the volume of a sphere, the area of a circle and the volume of a cylinder. “Those relationships form the basis for how stiff a structure is, how it will vibrate, and understanding how a design might fail,” says Charles Dandino, an engineer at the Jet Propulsion Lab.

“In my career, pi has allowed me to calculate the size of a shield needed to enter the atmosphere of Venus and the size of a parachute that could safely land the Curiosity rover on the surface of Mars,” says another JPL engineer, Anita Sengupta. Another handy tool that makes use of pi? The GPS system in your car and smartphone use it to calculate specific locations on Earth.

Toothpicks on the Turquoise blue background
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Do a magic trick to calculate pi

To try this easy activity known as Buffon’s Needles, you’ll need a large sheet of paper, at least 30 toothpicks, a ruler and a pen. Using a toothpick to determine the distance between them, draw a series of parallel lines on your paper. Then throw the toothpicks onto the paper at random.

Next, take away any toothpicks that are only partially on the paper, or that didn’t land on the paper at all. Count how many are left on the paper. Also count how many cross a line.

Divide the total number of toothpicks by the line-crossing toothpicks. Now multiply by two, and you should get pi!

Video camera lens lit in red and blue
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Pi is a movie star

Pi has made several significant appearances in pop culture. Most notably, there is an entire film about it called Pi: Finding Faith in Chaos. The 1998 film, which won an Independent Spirit Award for screenwriter Darren Aronofsky, is admittedly a bit of a downer, as it follows a tormented mathematician trying (and failing) to work out the secrets of the universe.

In the Sandra Bullock thriller The Net, clicking on a pi symbol is what sends Bullock’s character into the confidential government files, and in Alfred Hitchcock’s Torn Curtain, the pi symbol is the code that represents the escape network. It’s even vanquished TV villains; in a Star Trek episode, Spock outsmarts an evil computer by challenging it to calculate the final digit of pi.

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A pi-themed crop circle left scientists scratching their heads

In 2008, a 150-diameter crop circle suddenly appeared by Barbury Castle, near the English village of Wroughton. Researchers and conspiracy theorists puzzled over its origin and meaning, and an astrophysicist eventually figured out that the image was a code representing the first ten digits of pi.

smiling Sun Drawn with chalk On a Blackboard
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Pi makes people laugh

As it’s a monosyllabic word that also happens to sound exactly like a yummy dessert, there’s no shortage of jokes and puns to be made about pi. “What kind of reptile do math teachers keep as pets?” “Pi-thons!” “Why should you never start a conversation with pi?” “Because it goes on forever!” And the best part is, telling pi-themed jokes makes you sound smart while you’re making people laugh. Who says math can’t be fun?!

Sunset over distant empty highway
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Pi can go the distance

Here’s one way to picture how long pi is: If you printed out pi to a billion decimal values, in 12-point font, you’d need a piece of paper that stretched halfway across the United States, from Kansas to New York City.

flat lay with water bottle, wireless earbuds, smartphone, sneakers, and tshirt
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Celebrate Pi Day with a 5K

Besides being a nice rhyme, a 5K is pretty close to 3.14 miles in length (it’s 3.10686 miles, to be exact). Some schools and communities around the country host 3.14 mile runs on or near Pi Day every year.

slice of pumpkin pie in a tin with the pi symbol carved into it
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Better yet, celebrate pi with pie

It’s already a Pi Day tradition: pie in all its glory! Pizza pie, pot pie, shepherd’s pie and of course, dessert pie. Just make sure it’s round!

Tau of the Greek Alphabet
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Pi’s double has some fans, too

It would seem that every massively popular thing inevitably has some detractors, too, and pi is no exception. A growing number of math lovers have begun to argue that a number called “tau,” which is two times pi, deserves at least as prominent a place in the math world as pi.

Tau, whose approximate value is 6.28, is the relationship between the circumference of a circle and its radius, while pi relates the circumference to the diameter. Since the radius is a more useful mathematical quantity, many believe that tau is a more intuitive mathematic value than pi.

Opened book on top of stack of blue books
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Pi inspires poetry

One of the most interesting facts about pi (and Pi Day) is that you can also observe Pi Day with a poem… or a “piem”! Pilish unites math enthusiasts and word nerds. To compose in it, you must use words in which the number of letters corresponds, in order, to the numbers in pi’s sequence. So since pi = 3.14159, your poem must start with a three-letter word, then a 1-letter word, then a 4-letter word, another single letter and so on: “Aha, I said, a fancy alligator …”

Pi Day Pizza with the pi symbol drawn with pepperoni
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Pi Day inspires awesome Pi Day deals

All across the country, local and national stores typically celebrate Pi Day with sweet (and savory!) deals. For instance, you can get a pizza pie from Blaze Pizza at a special Pi Day price.


Catherine Holecko
Catherine Holecko is a contributor to RD.com’s Advice and Culture sections, where she writes about parenting, relationships, and pets. You can also read her work online at Highlights, Parents, and Verywell, and listen to her daily podcast at parentingroundabout.com. Catherine earned her BA in English at the University of Pennsylvania.