What Is Pi?

Pi is a circle's circumference divided by its diameter. (The diameter is twice the radius or double the length from any point on the circle to its center. The circumference is the distance around a circle.)

But what's remarkable is that no matter the size of the circle you are measuring, that ratio of circumference to diameter will always equal 3.1415926535897, usually shortened to 3.14.

Divide the circumference of a tennis ball by its diameter and you get 3.14. Divide the circumference of the planet Mars by its diameter and you get 3.14. Divide the circumference of the known universe by its diameter (assuming it's a perfect circle or sphere) — you get the point. As one mathematician put it, "Pi is part of the nature of the circle. If the ratio was different, it wouldn't be a circle."

This figure shows how the circumference of a circle with a diameter of 1.27 inches (32.35 millimeters) is equal to a linear distance of 4 inches (10.16 centimeters):

As you might imagine, 4.0 (the circumference) / 1.27 (the diameter) = 3.14.

Both the Babylonians and the Ancient Egyptians tried to estimate pi. Back in 1900 B.C.E., the Babylonians dabbled in the circle's circumference and decided that pi equals approximately 3.125. Meanwhile, the Ancient Egyptians pitched in with their own rough numerical approximation: 3.1605.

Enter the Mathematicians

But it wasn’t until the Greek mathematician Archimedes and the Chinese mathematician Zu Chongzhi stepped onto the scene that pi's estimation truly got refined to the most accurate approximation before calculus and supercomputers gave us the definitive answer [source: Exploratorium].

Symbolism and Pi's Universal Adoption

In 1706, the British mathematician William Jones made a choice that would become iconic. Jones assigned the Greek letter π to this wondrous number, perhaps because π is the first letter of the Greek words for periphery and perimeter.

Leonhard Euler, an 18th-century Swiss mathematician, took this symbolism to heart, popularizing its use. However, π wouldn't become a standard mathematical notation worldwide until 1934.

Pi in the Modern Era: From Tattoos to Holidays

The fact that you can find pi everywhere — not only in circles, but in arcs, pendulums and interplanetary navigation — and that it's infinitely long has inspired a cult following that includes plenty of geeky tattoos. In fact, it even has its own national holiday.

The Roots of the Holiday

The U.S. Congress might have officially recognized National Pi Day in 2009, but the roots of the holiday trace back to 1988.

Enter Larry "The Prince of π" Shaw, a beloved figure at the Exploratorium, San Francisco's renowned science museum [source: Exploratorium]. After the passing of the Exploratorium's founder, Frank Oppenheimer, Shaw crafted the idea of "π Day." The date, March 14 (or 3.14), fittingly echoes the initial digits of pi.

Even better, March 14 is also Albert Einstein's birthday, making π Day the ultimate geeky double-header.

The first π Day celebration was nothing more than Shaw and his wife handing out slices of fruit pie and tea at 1:59 PM (the three digits following 3.14), but the holiday quickly gained fame in the Bay Area.

Shaw eventually built the "Pi Shrine" at the Exploratorium, a circular classroom with a circular brass plaque at its center. Every Pi Day celebration at the Exploratorium ended with a colorful parade led by Shaw blasting his boombox (with a remix of "Pomp and Circumstance" set to the digits of pi) and circling the Pi Shrine exactly 3.14 times. The parade ended with the singing of "Happy Birthday" to Albert Einstein.

Celebrating π Day Worldwide

The Prince of π passed away in 2017, but the annual Exploratorium party continues, as do π Day celebrations the world over. How can you celebrate this quirky day?

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.