Working with Markups > Measurements > 3D Measurements > To Measure Length
  
To Measure Length
1. Click Markup > Length. The Measure Length dialog box opens.
2. Select a reference entity. The measurement appears in the graphics area and in the Results panel of the dialog box.
3. Press CTRL, and then select a second reference. The measurement of the second reference appears in the Results panel of the dialog box. The sum of the measurements appears in the Results panel and in the graphics area.
4. Click . The measurement is saved to the annotation set.