Explanation:
you need either:
the diameter- the distance from one side to the other, or
the Radius- the distance from the center to the edge
the diameter is always double the radius, and accordingly the radius is always half the diameter.
so, the circumference ca be figured out by multiplying the diameter times pi- which, for most calculations is 3.14
if you only have the radius, multiply that by 2 to get the diameter, then by 3.14.
hope that helps!