Diameter is the length of a line touching two points on a circle that passes through the center. Diameter exists only for circular, or for circle-based objects like a sphere or cylinder, thus the width and length should always be the same. Regardless what information you are given, you can find out the diameter if you have either the radius, circumference or area of the circle.
The diameter is the same as the length or width of any circle. No further calculation is required.
Calculating Diameter From the Radius
The radius is the length from the center of a circle to the edge. Therefore, if you know the radius, multiply it by two to determine the diameter (diameter = 2 x radius).
Calculating Diameter From the Circumference
If you know the circumference, you can divide the circumference by pi and this will be your diameter (diameter = circumference / pi). Pi is approximately rounded to 3.1416.
Calculating Diameter From the Area of a Circle
If you are given the area of a circle, the diameter is equal to the square root of four times the area divided by pi (diameter = √ (4 x area) / pi).