A web page and interactive applet show how to compute the area of an annulus. Text on the page explains that the area is the overall area minus the area of the circular 'hole' in the center. For those who prefer it in a formula that is given. The user can resize the two circles defining the annulus and visually estimate the area from a unit grid drawn inside it.

Applet can be enlarged to full screen size for use with a classroom projector. After use in the classroom, student can access it again from any web browser at home or in the library with no login required.

This resource is a component of the Math Open Reference Interactive Geometry textbook project at http://www.mathopenref.com.