What is pi? Simple geometry – Math, circles, circumference, diameter

Home » What is pi? Simple geometry – Math, circles, circumference, diameter
Print Friendly, PDF & Email
Parts of a circle

What is pi? The geometry of a circle

What is pi?

Pi is a number that is just a little bigger than 3.14. It is the number you get if you divide the circumference of any circle by its diameter. It’s the same for all circles. You can approximate pi for yourself by taking some circular things like the tops of jars and CD’s and frisbees and measuring their diameter and their circumference. When you divide the circumference by the diameter, you’ll get an answer something like 3.14. It will be the same every time (unless you measured wrong).

Pi is an irrational number

Actually, 3.14 is only approximately equal to pi. That’s because pi is an irrational number. That means that when you write pi as a decimal it goes on forever, never ending and never repeating itself. The first six digits of pi are 3.14159, and that’s all you need for most practical purposes. In most cases just 3.14 is enough.

A million digits of pi

Check out this webpage with a million digits of pi.

Why is this number called pi? History of pi

Usually in math we write pi with the Greek letter π, which is the letter “p” in Greek. You pronounce it “pie”, like apple pie. It is called pi because π is the first letter of the Greek word “perimetros” or perimeter. But it was not the Ancient Greeks who first discussed the value of pi. Mathematicians in the Babylonian Empire, about 2000 BC, had already figured out that pi was about 25/8, or 3.125. By about 1700 BC, in the Middle Kingdom, Egyptian mathematicians calculated pi to be about 3.16. Archimedes calculated that π was a little bigger than 3.1408. People have been gradually getting closer ever since, with early contributions from mathematicians working in ChinaIndia, and the Islamic Empire. By 263 AD, the Chinese mathematician Liu Hui had calculated that pi was 3.141.

Learn by doing: circumference of a circle
More about Circles

Bibliography and further reading about pi:


More about Geometry
More Easy Math
Quatr.us home

By |2018-04-23T16:01:03+00:00July 29th, 2017|Math|8 Comments
Cite this page: Carr, K.E. What is pi? Simple geometry – Math, circles, circumference, diameter. Quatr.us Study Guides, July 29, 2017. Web. October 16, 2018.

About the Author:

Dr. Karen Carr is Associate Professor Emerita, Department of History, Portland State University. She holds a doctorate in Classical Art and Archaeology from the University of Michigan. Follow her on Instagram, Pinterest, or Facebook, or buy her book, Vandals to Visigoths.


  1. Justine September 30, 2018 at 10:07 pm - Reply

    What is the Center Pi? Anyone knows the answer?

  2. za dank memes March 16, 2018 at 7:09 am - Reply

    very nice!

  3. emily March 14, 2018 at 11:34 am - Reply

    pi is equal to 3.14

    • Karen Carr March 14, 2018 at 12:34 pm

      No, it’s not. Pi is approximately 3.14, but it is an irrational number that continues infinitely, more and more decimals, forever, without repeating itself. A more accurate number is 3.1415923, but even that is only an approximation good enough for most practical purposes.

  4. bobett March 14, 2018 at 11:04 am - Reply

    how do you get the area of pi

  5. bob March 14, 2018 at 9:23 am - Reply

    this was very helpful what sa do

    • Karen Carr March 14, 2018 at 9:40 am

      I’m glad we could help!

Leave A Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.