Annulus (mathematics)

Of a circular ring is defined as the area between two concentric circles, that is. Between two circles with a common center Its surface area is


Wherein π is the circular number, and R and r are radii (respectively D and d, the diameter) of the mean outer and the inner circle.

The area can also be calculated from the inner and outer diameter and ring width:

With D and d is the diameter of the outer and inner circle and the ring width b. This information can be found eg in tube cross sections, while b is the wall thickness.