Area of an annulus
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
which simplifies a little to:
 

Try this

  1. In the figure above, click on "hide details"
  2. Drag the orange dots on the edge of the circles to make a random-size annulus.
  3. Now try to estimate the area of the annulus just by looking at the squares inside it
  4. Calculate the area using the formula
When you done click "show details" to see how close you got.

Other circle topics

General

Equations of a circle

Angles in a circle

Arcs