Pi Approximation Day


Pi Approximation Day celebrates the pi symbol (π) and its importance to mathematics. The symbol is a Greek letter, and it is used in mathematics to represent the ratio of the circumference of a circle to its diameter. The first digits of this ratio are 3.14159, and the number continues infinitely without a pattern, being an irrational number. It has been calculated to over one trillion digits after the decimal point. The symbol "π" was first used in 1706 by William Jones. It gained in popularity after being used by Swiss mathematician Leonhard Euler in 1737.

Pi can be expressed as the fraction 22/7, which is an approximation of its actual value (hence the word "approximation" in the day's title). Pi Approximation Day is particularly celebrated in countries that write their dates in the date/month format, where today's date would be written as 22 July or 22/7. Those who write their dates in the month/date format are more likely to celebrate Pi Day. However, both holidays can be celebrated no matter where one lives. Pi Approximation Day may have first been celebrated at the Chalmers University of Technology in Gothenburg, Sweden.

Pi Approximation Day, also known as Casual Pi Day, is observed next on Wednesday, July 22nd, 2020. It has always been observed annually on July 22nd.

How to Observe

Here are a few ideas on how to celebrate the day:

Occurrence Patterns

ObservedFirst YearLast Year
annually on July 22nd--



Alternate Names

  • Casual Pi Day

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