Question

How can I find diameter of a circle?

Answer 1

You, probably, are asking about constructing a diameter if a circle is given.
See below.

Explanation:

Let's use a theorem that a perpendicular from a center of a circle onto any chord divides this chord in half (you can see the proof at Unizor following the menu items Geometry - Circles - Mini Theorems 1).

Choose any pair of points on a circle, draw a chord between them and draw a perpendicular bisector to this chord. It will pass through a center and, therefore, its part inside a circle, from one intersection with a circle to another, is a diameter.