Finding the center of a circle using any right-angled object
An easy way to find the center of a circle using any right-angled object. Here we use a 45-45-90 drafting triangle,
but anything that has a 90° corner will do, such as the corner of a sheet of paper.
Click on 'Next' to go through the construction one step at a time, or click on 'Run' to let it run without stopping.
This page shows how to find the center of a circle using any right-angled object. This method works as a result of using
in reverse. The diameter of a circle
to any point on the circle. By placing the 90° corner of an object on the circle, we can find a diameter. By finding two diameters we establish the center where they
Printable step-by-step instructions
The above animation is available as a
printable step-by-step instruction sheet, which can be used for making handouts
or when a computer is not available.
Why it works
This method works as a result of Thales Theorem. The
diameter of a circle
right angle to any point on the circle. The converse is also true:
A right angle on the circle must cut off a diameter. By finding two diameters, we find the center where they
Visit Thales Theorem for an animated description of how this works.
Other constructions pages on this site
Circles, Arcs and Ellipses
(C) 2009 Copyright Math Open Reference. All rights reserved