The number of square units it takes to completely fill an
annulus.

Try this Drag the orange dots to move and resize the circles. As the size of the circles
change, the area is recalculated.

Area formula

The area of the
annulus
is the area of the ring-shaped space between the two circles that define it.
In the figure above it is the number of gray square units it takes to fill the annulus.
This area is the area of the entire disk, minus the area of the 'hole' in the middle. ( See Area of a Circle):

where: R is the radius of the outer circle H is the radius of the inner 'hole' π is Pi, approximately 3.142