If you are given the radius of the circle, you can use the formula: diameter = 2*radius If you are given the circumference of the circle, you can use the formula: diameter = circumference/pi
divide diameter by 2
C=Pi x diameter
Circumference = (diameter) multiplied by (pi)
Circumference of a circle = (pi) x (diameter)
circumference=pi * diameter radius= 1/2 diameter
Radius is half of the diameter, so you can easily come up with a formula: L = 0.25 * pi * d2
diameter = circumference/pi (pi = 3.14159265 or 3.14 for rough approximation).
in place of r (radius) write d/2 where d is diameter
Circumfrence=diameter X 3.14 (diameter=2 X radius)
That depends what shape you are talking about.