The easiest way to determine the diameter (D) circumference can be used in the case when you know the radius (R) of the circle. By definition, the radius is the segment connecting the center of the circle with any point lying on the circle. From this it follows that the diameter be two sections, the length of each of which is equal to the radius: D=2*R.
Use to calculate the diameter (D) ratio called PI, if you are aware of the length of the perimeter (L). Perimeter for a circle is called the circumference and the number PI expresses a constant ratio between the diameter and the length of a circle in Euclidean geometry, the division of the perimeter of a circle to its diameter always equals PI. Means for finding the diameter and circumference you need to divide this constant: D=L/π.
From the formula of finding the area of a circle (S) it follows that to find the diameter (D) you should find the square root of the result of dividing the area by PI and double the value obtained is: D=2*√(S/π).
If the circle described by the rectangle and the length of its side is known, there is nothing to calculate there is no need in such a rectangle can be a square, and the length of its side is equal to the diameter of the circle.
In the case of circle inscribed in a rectangle the length of the diameter is the same as the length of its diagonal. To identify it with the known width (H) and height (V) of the rectangle, you can use the Pythagorean theorem since the triangle formed by the diagonal, width and height will be rectangular. From theorem it follows that the length of the diagonal of the rectangle, and hence the diameter of a circle is equal to the square root of the sum of squares of width and height: D= √(H2+V2).