Annulus
The area between two concentric circles.
formula
\(A= \pi r_{id}\;^2 - \pi r_{od}\;^2 \)
\(A= \pi \left( r_{id}\;^2 - r_{od}\;^2 \right)\)
Where:
\(A\) = area
\(\pi\) = Pi
\(r_{id}\) = radius (inside diamester)
\(r_{od}\) = radius (outside diameter)