Working with Markups > Measurements > 3D Measurements > To Measure Area
  
To Measure Area
1. Click Markup > Area or right-click the graphics area and choose area from the shortcut menu. The Measure Area dialog box opens.
2. Select a reference entity to measure. The measurement appears in the graphics area and in the Results panel of the dialog box.
3. Press CTRL+Select a second reference entity. The second measurement appears in the Results panel of the dialog box. The total area of the references appears in the Results panel and in the graphics area.
4. Click . The measurement is saved to the annotation set.