The Measure Distance tool lets you quickly calculate the distance between two points, using just two clicks. The calculated distance is displayed in display units in the status bar.

To measure the distance between two points:
Using the Default menu system, choose Tools menu > Measure Distance.
You can also open the Extras toolbar and click
(Measure Distance).
Click in the viewport where you want to start measuring from.
Click again in the viewport where you want to measure to.
A distance value is returned in the status bar. Additional information, detailing the distances along the X, Y, and Z axes, is displayed as well.