 Oksana Mizina // Shutterstock
Happy Pi Day! What the number is actually used for
Mystical, magical pi has captured the imaginations of mathematicians for almost 4,000 years. An irrational number that stretches to infinity and beyond, pi is more than just an esoteric mathematical puzzle—it’s a reassuring constant that regulates the patterns and rhythms of the natural world around us, as well as within us.
Merriam Webster defines pi as both the symbol (π) representing the ratio of the circumference of a circle to its diameter, as well as the ratio itself—a transcendental number generally rounded to 3.14159265. The Babylonians and the ancient Egyptians were both conscious of the concept of pi in its most basic expression. Around 250 B.C., the Greek mathematician Archimedes of Syracuse extended pi to 3.141. In the early 1990s, the Chudnovsky brothers, using a computer cobbled together in their cramped Manhattan apartment, calculated pi to more than 2 billion digits. More recently, supercomputers have pushed pi past the 22 trillion digit mark.
In honor of international Pi Day—the 14th day of the third month of the calendar year—Stacker has isolated 20 of the most accessible uses of pi in everyday lives. So whether you choose to celebrate with cherry, pizza, or chicken pot pie, rest assured: On Pi Day you can have your pi and eat it, too.
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 1 / 21National Portrait Gallery // Wikimedia Commons
Birth of pi
Pi is used to determine the area of a circle. Pi's importance, however, extends far beyond that simple calculation. It stars in a number of other, larger mathematical equations, including Euler's identity and the Fourier transform, both of which have had a tremendous impact on the fields of science, technology, and engineering. The snappy symbol π, derived from the Greek alphabet, was introduced by Welsh mathematician William Jones in the early 18th century.
 2 / 21Africa Studio // Shutterstock
Piping hot pi
Pi is an essential element of the Fourier transform, developed in 1822 by French mathematician JeanBaptisteJoseph Fourier to measure the movement of heat waves. The equation continues to inform the design of modern heating systems. Scientists at the University of Maryland recently created a selfpowered thermoregulation fabric, which instinctively warms the body, vetting the material's effectiveness with a Fouriertransform IR spectrometer.
 3 / 21Connect world // Shutterstock
Pi and DNA
Pi can be used to measure one of the most basic components of human life—DNA. The ladderlike structure of DNA contains a mindboggling amount of genetic information, typically measuring 1.8 meters long. Amazingly, this string manages to wrap itself into a tiny, 10micron sphere—approximately 1.8 pi of its original length.

 4 / 21B_Me // Pixabay
Population proportion
What proportion of the American population has brown eyes? Owns their home? Can speak a foreign language? All of these questions can be answered with the help of the Greek letter π (pi), commonly used by statisticians when calculating population proportions.
 5 / 21Iwtt93 // Flickr
Rolling on a river
In 1926, Albert Einstein noted that rivers often travel in loops, which are similar to circles. As with all things that curve or bend, pi appears to inform the length of meandering rivers, known as sinuosity. In 1996, HansHenrik Stolum built on Einstein's observations, determining that the sinuosity of the average river trends toward 3.14, or pi.
 6 / 21Jerome Bon // Flickr
Pi and the Pyramids
In 1859, British writer and theorist John Taylor realized that the perimeter of the Great Pyramid at Giza, when divided by its height, equals 2pi. Like their ancient Egyptian predecessors, modern architects and engineers also rely on pi when calculating curved elements, such as pillars and pipes.

 7 / 21Photopictures // Shutterstock
Siesmic pi
Vibrations—including those created by earthquakes—travel in waves, and can, therefore, be measured by the Fourier transform. Engineers and architects can consult data gleaned from Fourier Transform infrared microspectroscopy to design buildings capable of withstanding the potentially disastrous effects of shifting tectonic plates.
 8 / 21Nicky Pe // Pixabay
Electric pi
The drudgery associated with household chores is decidedly less onerous thanks to modern electrical appliances—all of which owe their existence to pi. Pi is critical for the computation of electrical currents and their accompanying wave patterns. The larger the wave pattern, the bigger the wire needed to contain the power coursing through it.
 9 / 21Batistaya // Wikimedia Commons
Pi plays ball
Whether you're knocking it out of the park, dunking it through a hoop, or slamming it cross court, a ball is essentially a simple sphere. As with the manufacture of any spherical object, its surface area, which determines the amount and cost of the materials necessary for production, is calculated using pi.

 10 / 21skeeze // Pixabay
Computing pi
It took a supercomputer to calculate pi to a jawdropping 22 trillion digits, yet without pi, which is used to calculate the area of microchips, computers never would have existed. Engineers also rely on pi to determine the size and shape of a computer, as well as necessary air flow.