How do you find the radius and diameter of the circle?

Answer 1

The radius is half of the diameter.

The diameter is double the radius.

If you don't have either one then it can be pretty complicated to get an accurate measurement.

If you know where the center of the circle is, then you can draw any line through the center, and

measure the distance between the two points where the line hits the curved part of the circle.

But if all you have is the circle, with no point to show you where the center is, then there's no

simple method for measuring it precisely.

You can bisect two chords, and the bisecting lines will intersect at the center, or you can just measure the greatest width (although this is not so accurate.)

See the related link, The Parts of the Circle, below this answer.