How to Find the Centre of a Circle

The POSH way

  • Draw a chord anywhere across your circle (marked in yellow on the picture)
  • Construct the perpendicular bisector (marked in blue on the picture).
  • Draw another chord and do the same thing.
  • Where the bisectors cross will be the centre.


1/ Get a circle 2/ Get a book that's at least as big as the circle. (If you haven't got a book you can use a rectanglular piece of paper). 3/ Put one corner of the book on the edge of the circle (See "C") Mark where the two edges of the book cross the circle (See "X" and "X")
4/ Remove the book and join the marks with a straight line. This will be a diameter of the circle 5/ Do it again to get another diameter. 6/ Where the diameters cross is the centre.

This works thanks to the fact that the angle in a semi-circle is always a right angle. There's more of this stuff in Savage Shapes

You might also like How to find the centre and foci of an ELLIPSE!


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