If the radius of the circle is known, the diameter formula is diameter=2R, where R equals the circle's radius. If the circumference of the circle is known, the diameter is found using diameter=C/Pi, where C is the circumference and pi is approximately 3.142.

If the area of a circle is known, the diameter is the square root of 4A/Pi, where A is the area of the circle and Pi is approximately 3.142. The diameter of the circle is the length of the line through the center of the circle that touches two of the points on its edge The center of the circle is the midpoint of the diameter. A diameter may also be referred to as a chord, which is any line that joins two points on a circle. The diameter is the longest chord on any circle.