The formula for the circumference c of a circle, in terms of the radius, is c = 2ttr, where r stands for the radius. rearrange the quantities in this formula to give a new formula for the radius of a circle in terms of the circumference.

r = (C / π) / 2

Actually the formula of the circumference is Pi multiplied to diameter; where the diameter is equal to twice the radius.

C = π d
d = 2 r
C = π * 2r

If we need to find the radius from the Circumference then

r =  (C / π) / 2

You must differentiate Pi from the number 2 because 2 is part of the radius. This is also to avoid confusion.

For example:  r = 5 ; π - 3.14; find the circumference

C = π * 2r
C = 3.14 * (2*5)
C = 3.14 * 10
C = 31.40

If the circumference is given and the radius is missing, then,

r = (C/π) / 2
r = (31.4 / 3.14) / 2
r = 10 / 2
r = 5