You need to measure your Word document in centimeters, not inches? You can change your ruler to centimeters, millimeters, points (used for pixels) or picas, along with inches.
Click File > Options > Advanced and scroll to the Display settings. Look for Show measurements in units of, and change the units.
Both the vertical and horizontal rulers will change. You see the vertical ruler only in the Print Layout view. You can also show or hide rulers.