The distance tool or measurement tool is a small utility that helps measure distance between two points in 3D space. 

 

1. When you click on "Tools", then "Distance Tool" and hover your mouse over any 3D content in the step (or just the grid itself), a cyan dot will start following your cursor. This dot is magnetic to the closest vertex nearing your mouse pointer.
2. Place that dot with a left mouse click, to determine the starting point of the measurement.
3. Then click on a second vertex to display the distance between the two points.
4. Upon finalizing the location of the second dot (with left mouse click) the tool will remain in the step for the remainder of the session, or until it gets removed.
5. The start/end points can also be moved around by dragging them after their initial placement. To do so, first select a point, then move your mouse to the new location, then click again to confirm its updated placement.

Note:

• To deselect the measurement, you can press "esc"
• To delete the measurement, first select the teal label. You can tell it's selected if the x, y and/or z coordinates are displayed as well. Then press "Delete".
• You can select your preferred unit of measurement by choosing any of the available units in the preferences menu.
  

Tip:

• You can also use the dimension widget to evaluate the size of an object.
  The widget will show you the bounds dimensions of a selected object (Selection Bounds) on the top right of the Scene View.
  
• It's possible to enable/disable and define the precision of this widget in Edit > Preferences > General > Scene View.