Diameter of a circle – Geometry

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Parts of a circle

The geometry of a circle

The diameter of a circle is the distance across the circle at the widest part of the circle. The diameter is always equal to twice the radius of the circle, because the radius starts from the center of the circle and goes straight to the edge.

Learn by doing: measure the diameter of some circles in your house
Area of a Circle
More about Geometry

Bibliography and further reading about circles:


More Geometry
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By |2017-07-29T09:34:48+00:00July 29th, 2017|Math|2 Comments
Cite this page: Carr, K.E. Diameter of a circle – Geometry. Quatr.us Study Guides, July 29, 2017. Web. August 21, 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. Phil January 20, 2018 at 6:47 am - Reply

    If a snooker ball has a diameter of 2.5 cms how many points of contact when colliding with a ball of exactly the same size are there?

    • Karen Carr January 20, 2018 at 2:09 pm

      That sounds like your homework? I’m sure you will learn more from doing your own homework than from having me tell you. Besides, just draw two circles next to each other to see the answer.

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