The circumference of a circle is given by the general formula;
C = 2 * π * r .... or C = π d (r = radius, d = diameter)
We can easily modify the second formula to isolate π in order to express the circumference in relation to the diameter:
π = C / d
To obtain π, you need to divide the circumference by the diameter.
We're used to think about the circumference in terms of radius... but since we deal with 2 * r * π and that 2 * r = d... it's also convenient to use the diameter.
