Whether you are trying to find the center of a circle for geometry class, a craft project or home repairs, you can employ the same technique. You will need to find three points on the circle's circumference and take perpendicular bisectors of the cords you form. You can perform the operation using ruler measurements or employing a ruler and compass.
Things You'll Need
 Ruler
 Compass
 Protractor

Mark three points anywhere on the circumference of the circle. Designate these points A, B and C. It doesn't matter where you put these points, but the farther they are from each other, the more accurate your calculations will be

Draw a line connecting points A and B. Call this line AB. Do the same with points B and C to create line BC.

Find the center of line AB and BC. You can either measure the distance and divide it in half or use geometry and bisect the lines. You can use a compass set at a distance greater than half the length of the line segment and make arcs above and below the line segment. Do this from both endpoints to form points above and below the line segment, then connect these points to divide the line segment in half. Mark these middle points D and E.

Draw perpendicular lines from points D and E using the compass technique or using a protractor to find a 90degree angle. Where these lines intersect is the center of the circle.
Tips & Warnings
 You can check that you have found the center of the circle. Simply measure from the point you found to the edge of the circle. Pick a few different points. If they are all the same distance, you have found the center.
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References
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