An illustration of a circle showing the circumference, centre, diameter and radius.
The 'Ci'rcumference of a closed, rounded shape is the same thing as the perimeter of a circle. It is found by multiplying pi times the diameter of a circle.
Circumference of a circle
The circumference of a circle is the length around it. The circumference of a circle can be calculated from its diameter using the formula:
Or, substituting the radius for the diameter:
where r is the radius and d is the diameter of the circle, and the Greek letter π is defined as the ratio of the circumference of the circle to its diameter.