You may have the ruler marked in one of several scales; one of inches, millimetres, or centimetres. Inches may be divided in quarters, eighths, tenths or sixteenths.
To choose the measurement:
Display the Extra menu, and choose Ruler or Side ruler, depending on which you want to change.
Select a measurement from the list.
The ruler will display your chosen measurement and it will also be the measurement used in dialogue boxes.
You can use the Choices dialogue box to set the default ruler measurement for new documents.