While there’s a food holiday for just about everything you love to eat, it turns out there are actually *two* separate days every year when people come together to celebrate pie: National Pie Day on January 23 and Pi Day on March 14. As you can imagine, it’s easy to confuse the two occasions when there’s pie involved but here’s a quick breakdown of what makes them different. For starters, Pi Day is about much more than stuffing your face with flaky, fruit-filled desserts.

Now would be a great time to grab a generous slice of apple pie (and some Blue Bell ice cream on top) before we dig into what makes each of these two dates special. Learning makes you hungry, right? No? Just me? Okay.

**When Is National Pie Day?**

National Pie Day is observed on January 23.

**What Is National Pie Day?**

We can thank Charlie Papazian, an American nuclear engineer, craft beer brewer, and author who is credited with creating the food holiday in the 1970s after he declared his birthday to be National Pie Day. Really, he just up and proclaimed the date as a national celebration of pie. The American Pie Council has sponsored the holiday since 1986.

Of course, every day can be *pie day* if you try hard enough but some restaurants—mostly local restaurants and regional chains—end up marking the date with free pie, discounts on pie, and other pie promotions. Keep an eye out for deals at your favorite restaurants or you can always celebrate by baking a pie yourself.

**When Is Pi Day?**

Pi Day is observed on March 14.

**What Is Pi Day?**

Get ready for a flashback to high school math class.

Pi Day is celebrated on March 14 (3/14) around the world. Pi (Greek letter *π*) is the symbol used in mathematics to represent a constant—the ratio of the circumference of a circle to its diameter—which is approximately 3.14159. For any circle, the distance around the edge is a little more than three times the distance across. Pi Day is an annual opportunity for math enthusiasts to recite the infinite digits of Pi, talk to their friends about math—and eat pie.

Pi has been calculated to over 50 trillion digits beyond its decimal point. While only a handful of digits are needed for typical calculations, pi’s infinite nature makes it a fun challenge to memorize and to computationally calculate more and more and more digits.

Typing π into a calculator and pressing ENTER will yield the result 3.141592654, not because this value is exact but because a calculator’s display is often limited to 10 digits. Pi is actually an irrational number (a decimal with no end and no repeating pattern) that is most often approximated with the decimal 3.14.

**Pi: A perennial puzzle**

Pi has interested people around the world for over 4,000 years. Many mathematicians from famous ones such as Fibonacci, Newton, Leibniz, and Gauss to lesser well-known mathematical minds have toiled over pi, calculated its digits, and applied it in numerous areas of mathematics. Some spent the better parts of their lives calculating just a few digits. Here is a sampling of the many milestones in the life of pi.

Early decimal approximations for pi were obtained in a number of different ways. For example, in ancient Babylon rope stretchers marking the locations of buildings and boundaries estimated pi to be 258 = 3.125. The ancient Egyptians determined the ratio to be (169)2 ≈ 3.16. The earliest calculations of pi were largely based on measurement.

Archimedes of Syracuse (287–212 BC), a Greek mathematician, was the first to use an algorithmic approach to calculate pi. He drew a polygon inside a circle and drew a second polygon outside of the circle. Then he continuously added more and more sides of both polygons getting closer and closer to the shape of the circle. Having reached 96-sided polygons, he proved that 22371 < pi < 227.

However, it was first given the Greek letter as its name when William Oughtred called it as such in his works dating back to 1647 and later embraced by the scientific community when Leonhard Euler used the symbol in 1737.

**History of National Pi Day**

But how did Pi Day end up in a country-wide phenomenon? For that, we need to travel to the Exploratorium in San Francisco where in 1988 it was thought up by physicist Larry Shaw.

Shaw linked March 14 with the first digits of pi (3.14) in order to organize a special day to bond the Exploratorium staff together where he offered fruit pies and tea to everyone starting at 1:59 pm, the following three digits of the value. A few years later, after Larry’s daughter, Sara, remarked that the special date was also the birthday of Albert Einstein, they started celebrating the life of the world-famous scientist.

Pi Day became an annual Exploratorium tradition that still goes on today and it didn’t take long for the idea to grow exponentially hitting a peak on March 12, 2009 when the U.S Congress declared it a national holiday.

Now, celebrated by math geeks all around the circumference of the world, Pi Day became a pop culture phenomenon with several places partaking in the activities, antics, observations, and all the pie eating they can.

**By the numbers**

- 14
^{th}of March: Also Albert Einstein’s birthday - 31.4 million: The world record held by Emma Haruka Iwao for calculating the most accurate value of pi
- 4: The number of months it took Emma Haruka Iwao to calculate the most accurate value of pi
- 70,000: The number of decimal places of pi memorized by Rajveer Meena in 2015
- 10: The number of hours it took Rajveer Meena to make the world record
- 3.125: The original number used for pi by the Babylonians
- 22.4 trillion: The number of digits calculated by Swiss scientist Peter Trueb using a computer
- 24: The number of hard drives on the computer used by Peter Trueb for calculating pi
- 700,000: The number of years it would take to recite the 22 trillion digits of pi
- 15,000: The number of digits of pi memorized by Mark Umile in the U.S.

**How to celebrate National Pi Day**

Celebrate Pi Day wherever you are.

**1. Visit the Exploratorium**

The Exploratorium in San Francisco holds an all-day celebration. 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.

Visiting the Exploratorium? Look for the exhibits *Pi Has Your Number* and *Pi Toss*.

**2. Write a π-ku**

First line: 3 syllables

Second line: 1 syllable

Third line: 4 syllables

**2. 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.

**3. 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).

**4. Cutting Pi**

String and scissors are all you need to find pi all around you.

It may be hard to cut pie into equal pieces but in this Snack you can cut string into pi pieces.

**5. Pi Graph**

Use straight lines to learn about circles.

Graphing data can help you discover patterns in nature.

**6. Pi Toss**

Randomly toss some toothpicks, with pi as your reward.

Asked to get an estimate for the famed mathematical constant pi, you might do what the ancient Greeks did: Divide the circumference of a circle by its diameter. Here you can estimate pi by a less conventional method: the random tossing of toothpicks.

**7. Bake and eat a pi(e)**

Perhaps the greatest way to observe Pi Day is to bake—then eat—a pie with the pi sign baked into the upper layer of crust or lattice. You can also create the pi sign with pepperoni on top of a pizza. Or since Pi Day only happens once a year, you can go wild and make both!

How you celebrate National Pie Day and Pi Day is up to you. As always, celebrate responsibly.

**Worth Pondering…**

Probably no symbol in mathematics has evoked as much mystery, romanticism, misconception and human interest as the number pi.

—William L. Schaaf, *Nature and History of Pi*