There are a lot of theories about who actually started to use pi when trying to solve the circumference of the circle. Some say that it was first discovered in India in the 5th century. It was used by Aryabhata when he was 23 years old. Some say that it was first used by David Gregory in the year 1697.
it was also used by William Jones in the year 1706. The “official” person who has stated the value of pi is Archimedes of Syracuse. He is said to have done it during his lifetime which is from 287 - 212 BC. It is evident that other people agreed with him and this formula is still being used now.
I am not sure who theorized that pi was necessary in a circle. I have read that its importance was first recognized by the Babylonians and Egyptians as early as 2000 B.C. Maybe they needed pi to figure out how to make circles large enough to help build the pyramids?
Mathematicians throughout the years have calculated pi out to many hundreds of places but it has only been with the help of computers that we now know, as of December 2013, the first 12 trillion digits of pi. If you’re a pi lover, you might want to know that Pi Day is celebrated every year on Albert Einstein’s birthday, March 14.