This is a tool that calculates the area, semi-minor axis length or semi-major axis length of an ellipse.

The tool is multi-directional and, therefore, determines the information that needs to be calculated based on which data has been entered. In other words, if two out of the semi-major axis length, semi-minor axis length or area are entered, the tool calculates the remaining one. When further data changes are made, the tool re-calculates the same ellipse property unless that property is itself changed or a different data field is cleared.

Associated tool link: http://www.coggit.com/tools/dimensions_of_an_ellipse.html