To Measure the Area of the Difference Between Elements

Select the Measure Area tool ( Drawing > Analyze > Measure ).
In the tool settings window, set Method to Difference.
If any elements are selected, they are de-selected.
Select the element from which to subtract.
Select the element to subtract from the element selected in the preceding step.
(Optional) Use <Ctrl-data point> to select further elements to subtract.
Enter a data point.
The results display in the tool settings window. When measuring a union, the Perimeter value does not display if the elements do not overlap. If Mass Properties is on, the mass properties analysis is displayed in the Mass Properties window.