To calculate a circle's diameter from its circumference, one should divide the circumference by the constant pi, which is an irrational number approximately equal to 3.14159265358979.
Circumference is defined as the distance around a circle; the diameter is the straight-line distance across the circle that touches two points on its border and goes through its center. The ratio of a circle's circumference to its diameter is the constant pi. In other words, circumference/diameter = pi, or circumference = pi * diameter. Therefore, the length of the diameter of a circle is equivalent to its circumference divided by pi.
Similarly, the length of a circle's radius, (the distance from the circle's center to any point on its border), is equivalent to its circumference divided by (2*pi). The radius is half as long as the diameter.