Pi Day (aka Pie Day) is a magical day — it’s one on which both number nerds and pastry eggheads rejoice. At Yummly, that means everyone is in an excited state of reflection about pi... over pie. And since the official birthplace of Pi Day — the San Francisco Exploratorium museum — is just up the San Francisco Bay from the Yummly offices, we feel it’s our duty to give you an edible ode to numbers in the form of a short guide on pi(e), with ideas for celebrations and plenty of pie recipes.

What Does Pi Day Mean?

Before we get into how to make math delicious, we should explain what Pi Day is. In addition to being Albert Einstein's birthday, Pi Day happens every March 14 which, written numerically in the United States, is 3/14. In mathematics, pi (or 𝛑) is the ratio of a circle’s circumference to its diameter: The circumference of any circle is approximately 3.14 times its diameter (I say approximately because we typically only see 𝛑 in shortened numerical form as 3.14, but it’s actually never-ending).

That means the circumference of the 4 1/2-inch honey buttermilk pie that’s cooling on my kitchen counter right now is 14.13 inches (4.5𝛑 = 14.13 or 4.5 x 3.14 = 14.13). And the cookie butter pie setting in my refrigerator with a circumference of 28.6 inches is in a 9-inch pie plate (28.6 ÷ 3.14 = 9 or 28.6/𝛑 = 9).

We use the Greek letter 𝛑 (pi) as the symbol for this irrational number because it is the first letter in “perimetros” or perimeter, a synonym of circumference. We also get pie from the Greeks (though the ancient Egyptians did eat something similar). Pie as we know it started showing up on Greek tables around the 14th century. After that, this culinary concept spread to the Romans, then to the rest of Europe and beyond.

The Shape Of Pi(e)

With Greek influence coming at pi(e) from all angles, you might be wondering: Does pie have to be round? The answer is no, pie does not have to be round. A round pie might be expected on Pi(e) Day, but we like to think of it as an excuse to enjoy pie in all sorts of shapes, sizes, and flavors.

Slab Pies

A slab pie is just a rectangular pie, but it's great for serving a crowd (think: Fourth of July cookout). The dough is pressed into a sheet pan, filled, and baked or chilled. The filling is spread out a bit so it’s not as thick as a regular round pie, but it’s just as delicious. Slab pies are very easy to make if you’re not using supermarket pie dough. (Those are pre-cut into circles, so it wouldn’t work in a sheet pan.) If you want to know how to make perfect pie dough from scratch, you can check out our guide on how to make pie.

Tarts

The tart is the more elegant form of our favorite confection (it is French, after all). Tarts are pies that have a shallow shell so you have a more even crust-to-filling ratio (more ratios!). However, tart crusts can vary. Some bakers use a simple pâte brisée (regular pie crust), others use a pâte sucre crust (a sweeter shortbread crust), or even a chocolate cookie crust. A traditional French tart is filled with pastry cream (or crème anglaise if you want to say it in French), topped with fresh berries, and brushed with an apricot glaze, but you can fill it however you want.

Galettes

Galettes are freeform pies that have a rustic look. If you don’t happen to have a pie pan on hand, you can make one of these — it’s also probably the easiest way to make a pie if you’re into imperfect style: You roll out the dough into a disheveled circle, pile on the fruit and roughly fold up the edges for a country confection. Gallettes work well for fillings that aren’t particularly gooey, because they’re not as deep as conventional pies. Also, IMHO, they’re great for berry pies (especially blueberry) which in the form of a traditional deep-dish pie can be overwhelmingly fruity.

Pot Pies

Pot pies like chicken pot pie and beef pot pie are similar to the original pie that the Greeks came up with. It's the American name for savory pies with a sauce. In Australia, they're just called meat pies. They're sometimes called royal pies in the U.K., but they also have pork pies which is a little bit different because there's no sauce or gravy involved — it's minced pork pressed together and wrapped in pastry. But let's not nitpick; whatever you decide to call it, it means you get to eat pie for dinner!

Shepherd’s Pie

Shepherd’s pie is a savory pie made of meat topped with mashed potatoes which acts as the crust. Shepherd’s pie originated in the United Kingdom and is the same thing as cottage pie, however, some use the term “shepherd’s pie” to refer to pies made with lamb and reserve “cottage pie” to refer to pie made with ground beef. They’re eerily similar to casseroles, but we’re going to give these a pass since they’ve been called “pie” since at least the 1800s.

Pizza Pies

We’d have a very narrow definition of pie if we did not include a pizza pie, but we recognize that not everyone would agree with that categorization. In New York City, it’s common to refer to a whole pizza as a pie, though it’s not totally clear how that came about. Some say it’s derived from an ancient Italian tomato pie, others say it’s just because it's round like a pie. I’m dubious about either theory, but according to Harris Insights, it’s America’s second favorite pie (apple pie is THE favorite), so we (happily) had to include it.

Hand Pies

Hand pies have a few different names in addition to traditional, American hand pies, so here are a few descriptions and explanations.

Hand Pies There is, of course, the regular ol’ hand pie. Hand pies are made with a standard pie crust, only they’re a fraction of the size of a regular pie (they do have to fit in your hand!). You can cut dough into freehand rectangles and use two pieces to seal in the filling. Or if you’re a perfectionist, you can make them uniform by using a large cookie cutter or biscuit cutter. Either way, you’ve got a sweet hot pocket.

Turnovers Turnovers are slightly different from hand pies in that they are made with puff pastry. The puff pastry is cut into squares, filled, and then folded into triangles before baking (that's the kind of geometry I'm into!). They can be filled with anything, but apple turnovers are probably the most common. Many recipes also call for a drizzle of icing over the top.

Pop Tarts Technically, “Pop Tart” is a brand of toaster pastry, but there are enough copycat recipes for them that we thought we should make note of them. They are rectangular hand pies with a sweet filling (typically fruit) and icing.

Empanadas Empanadas are savory, moon-shaped hand pies that originated in Spain but they’re most commonly found in Argentina. The pastry is slightly different from common hand pies in that it calls for an egg mixed into the dough. In cafes around Buenos Aires, you’ll find them filled with ham and cheese, cheese and onion, or just plain cheese, but you can fill them with anything you want. Ground beef and ground sausage (like chorizo) work well.

Cornish Pasties Cornish pasties (pronounced “pass-tees”) are hand pies that originated in Cornwall, England. They’re a bit bigger and denser than the empanada, but the idea is very much the same: meat wrapped in pastry for a portable meal. These particular pastries are filled with beef, potatoes or turnips, and onions. Because they have Protected Geographical Indication status in Europe, the filling can’t be messed around with if you want to call them Cornish pasties.

Pi(e) Day Celebrations

At Yummly, we celebrate Pi(e) Day by making and eating pie, of course, but celebrating Pi Day has been a tradition at its birthplace at San Francisco’s Exploratorium since 1988. Physicist Larry Shaw started the party with pie and a parade back then and the tradition continues — there’s even a permanent plaque on the sidewalk outside the Exploratorium to honor the day. The holiday has exploded and expanded exponentially and people now celebrate it in nerdy ways all over the country.

Celebrate with numbers (and pie) At the Massachusetts Institute of Technology, March 14 has turned into an unofficial holiday when the school lets future admits know they've been accepted, and current students and alumni gather to eat pie.

Recipe for a song Because 𝛑 is a never-ending number, we typically only see 𝛑 written as 3.14 and we usually don’t see any digits beyond the “4.” Some mathletes make it a personal challenge to memorize an additional digit each year, but one guy took that idea a step further and composed a song based on the first 200 digits of 𝛑 (here’s an explanation).

Pi(e) fest At Harvest Middle School in Napa, California, they often treat Pi Day like a carnival, with a pie eating contest, hula hooping contest, a calculating contest, and students create 𝛑 art. It’s hard to tell where the fun ends and where it begins when you’re celebrating circles!

Round runs In Milwuakee, Wisconsin, they hold a 3.14 mile run through the city as a part of the annual celebration. Runners receive pie at the finish line.

Classic Dessert Pies

Those are just a few of the ways people are celebrating, but if you’re going to celebrate Pi(e) Day, we think you should start with delicious dessert pie recipes! And much like 𝛑’s infinite digits, the number of ways to make a sweet pie is infinite — and we’ve got a few thousand pie fillings to choose from. So get baking (and don't forget the ice cream or whipped cream).