Diameter is the length of a line touching two points on a circle that passes through the center. This line is most commonly known in mathematics as the diameter of the circle. Diameter exists only for circular or cylindrical objects, thus there is only one width or length, and these should be equal numbers. Regardless what information you are given, you can find out the diameter if you have either the radius, circumference or area of the circle.

Calculate the diameter. If you are given the length or the width, then the diameter is the length or the width of the circle. No further calculation is required.

Calculate the diameter if you are given the radius. If you know the radius, multiply it by two to determine the diameter (diameter = 2 x radius).

Solve for the diameter if you are given 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.

Solve to find the diameter if you are given the area. Find the square root of four times the area divided by pi (diameter = √ (4 x area) / (pi)).
References
 Photo Credit inner circle image by Michel Essiambre from Fotolia.com