where π is the irrational number 3.14159... and r is the radius of the circle. Circumference is like perimeter but for a circle; it is the distance around the boundary.
The circumference of a circle can be found by multiplying pi ( π = 3.14 ) by the diameter of the circle. If a circle has a diameter of 4, its circumference is 3.14*4=12.56. If you know the radius, the diameter is twice as large.