You can measure the horizontal distance between any two points in the viewport. When you click two points, Measure calculates the distance between them on the x-axis. The points do not have to be attached to any elements.
To measure a horizontal distance
1. On the 3D Access — Annotation tab, in the Utilities group, click the arrow next to Between Points.
2. Click Horizontal. The Measure dialog box opens.
3. Specify the context in which you want to measure.
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 horizontal line as a guide. The horizontal distance between the two points is displayed in the Distance Output box.
5. Continue specifying pairs of points, or click or to complete the command.
You can click OK to use the output from Horizontal as input for an interrupted operation.
Between Points.
Horizontal. The Measure dialog box opens.
or 
to complete the command.