Every year on March 14—Albert Einstein’s birthday—number lovers can raise a piece of pie to Pi (π) Day, an international celebration of one of math’s most famous constants: the irrational, never-ending number π, pronounced pi (3.14159 . . .). Exploratorium physicist Larry Shaw invented the playful celebration in 1988, and Congress declared it an official U.S. national holiday in 2009. The San Francisco museum continues its annual festivities with an all-ages Pi Day celebration on March 14, 2021.
Celebrate Pi Day wherever you are. From the pie feast to the pi procession, from π activities to pi gear, the Exploratorium demystifies some of our long-standing Pi Day rituals and offers insight into a few fun ways for anyone to celebrate this infinite decimal.
1. Estimate π with Toothpicks
Grab a box of toothpicks to recreate the method known as Buffon’s Needles, which allows you to calculate π just like 18th-century French naturalist (and gambler) Georges-Louis Leclerc, Comte de Buffon. Buffon was inspired by a then-popular game of chance that involved tossing a coin onto a tiled floor and betting on whether it would land entirely within one of the tiles. This method is absurdly inefficient—but you’ll also be surprised that you can calculate π this way.
2. Throw a Pi(e) Feast
Pi Day and pie go hand in hand; Shaw’s very first Pi Day celebration in 1988 included a pie feast for Exploratorium staff, with fruit pies and a tea urn. So show off your crafty skills on March 14, and go all out on audacious pie crusts and creations worthy of Pinterest fandom—or simply order your favorite pizza pi(e).
3. Explore π Activities
From cutting π with scissors to writing your own pi-ku—a haiku about π—here are five easy activities exploring π that you can do at home with minimal preparation.
What is π anyway? Divide any circle’s circumference by its diameter; the answer (whether for a pie plate or a planet) is always approximately 3.14, a number we represent with the Greek letter π. Keep calculating π’s digits with more and more accuracy—as mathematicians have been doing for 4,000 years—and you’ll discover they go on literally forever, with no pattern.
As of 2019, π has been calculated to 31.4 trillion digits. Test your memorization skills, and see how far you can go.
5. Join the Pi Procession
For as long as the Exploratorium staff has eaten pie on Pi Day, there has also been the ritual π procession, with a high-spirited crowd gathering around the museum to Elgar’s “Pomp and Circumstance,” circling the Pi Shrine 3.14 times waving the digits of π, and singing a rousing chorus of “Happy Birthday” to Einstein, before sharing slices of pie.
These traditions continue to this day in San Francisco and beyond. So put on your favorite π-themed outfit and join us online for a live celebration packed with pi trivia, pi art, and pi music, and enjoy a serving of pie wherever you are!