In order to measure the distance between two points, you must first set the measure context. See
Set the measure context.
Measuring the Distance between Two Points
You can measure the distance between any two points in the viewport. The points do not have to be attached to any elements.
To measure a distance:
1. On the 3D Access — Annotation tab, in the Utilities group, click the arrow next to Between Points.
2. Click Between Points. The Measure dialog box opens.
3. Specify the context in which you want to measure. See above, Setting the Measure Context.
4. Specify two points by entering coordinates in the user input line or by clicking in the viewport. After clicking the first point, use the graphical feedback line as a guide.
5. The distance between the two points is displayed in the Distance Output box.
6. Continue specifying pairs of points, or click or to complete the command.
You can click OK to use the output from Between Points as input for an interrupted operation.
Between Points.
or 
to complete the command.