You can hide or display rulers. By default, rulers are hidden. When the cursor is on the image, a thin line appears on each ruler to show the cursor’s position. You can display rulers in pixels, inches, or centimeters and change the color of the ruler.
To display or hide rulers:
Choose View > Rulers or press Ctrl + Alt + R.
To change ruler units and colors:
Choose File > Preferences > General Program Preferences.
Click the Units tab.
In the Display units drop-down list, select Pixels, Inches, or Centimeters.
Choose the ruler color by selecting either Black on white or Toolbar colors.
Click OK.