The answer depends on the formula for what: the radius, circumference, length of an arc, area, area of sector, area of segment: each one has a different formula.
divide diameter by 2
C=Pi x diameter
(pi)D
diameter = circumference/pi (pi = 3.14159265 or 3.14 for rough approximation).
That depends what shape you are talking about.
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)
Radius is half of the diameter, so you can easily come up with a formula: L = 0.25 * pi * d2
(pi)D
in place of r (radius) write d/2 where d is diameter
Circumfrence=diameter X 3.14 (diameter=2 X radius)
diameter = circumference/pi (pi = 3.14159265 or 3.14 for rough approximation).
You just multiply the given radius by 2. That is diameter is just double of the radius. This is universal formula.
That depends what shape you are talking about.