Well, diameter of a circle is two times the radius. It is calculated as 2 multiplied by radius of the circle i.e. 2r (where r is the radius).
http://answers.ask.com/Science/Other/how_to_calcul...

1. Measure from one side of the circle directly across to the other side, through the circle's center. If the circle is drawn to scale, this measurement will be the diameter. 2. Multiply
http://www.ehow.com/how_6011124_calculatediameter...

1 If you have the radius of the circle : Add the radius again, or multiply the radius by 2. Or, r2 = D 2 If you have the circumference : The circumference is divided by Pi (3.14),
http://www.wikihow.com/CalculatetheDiameterofa...

divide diameter by 2.
http://wiki.answers.com/Q/What_is_the_formula_for_...
