Pi, Greek letter (π), is the symbol for the ratio of the circumference of a circle to its diameter. Pi Day is celebrated by maths enthusiasts around the world on March 14th. Pi = 3.1415926535…

With the use of computers, Pi has been calculated to over 1 trillion digits past the decimal. Pi is an irrational and transcendental number meaning it will continue infinitely without repeating. The symbol for pi was first used in 1706 by William Jones, but was popular after it was adopted by the Swiss mathematician Leonhard Euler in 1737.

Learn About Pi

The Number Pi

Outline of a Circle and its Diameter Pi represents the relationship between a circle’s diameter (its width) and its circumference (the distance around the circle).

Equations that use Pi

Area of a Circle

The area of a circle is calculated using Pi and the radius of the circle. This formula inspired the joke “Pies aren’t square, they’re round!”

Volume of a Cylinder

To find the volume of a rectangular prism you calculate length × width × height. In that case, length × width is the area of one side, which is then multiplied by the height of the prism. Similarly, to find the volume of a cylinder, you multiply the area of the base (the area of the circle, which is pi × r²), then multiply that by the height of the cylinder.

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