You can change measurement units for the on‑screen rulers and for use in panels and dialog boxes; you can also change these settings at any time and temporarily override the current measurement units as you enter a value. By default, rulers begin measuring from the upper-left corner of a page or spread. You can change this by moving the zero point. (See Change the zero point.)
Changing the measurement units doesn’t move guides, grids, and objects, so when ruler tick marks change, they might not line up with objects aligned to the old tick marks.