Currently, five units of measurement are offered on the on-screen ruler.  They are

1. Centimetres
2. Inches
3. Pixels
4. Percent
5. Picas
   

To set the units, you must first bring up the Options Dialogue Box by right-clicking anywhere on the ruler and selecting the Options menu item or by using the Ctrl + O key combination. The units may be set using the radio-buttons in the Units group box.  Click the unit you want to use, say "Inches".  The units of the ruler should change to inches.  Click OK to close the Options Dialogue Box.

To set the units using the mouse wheel, simply rotate the wheel until the ruler displays the units you want.  This is the fastest way to set the units, since you do not need to bring up the Options Dialogue box.

Alternatively, use the F3 key to cycle through the units with each key press.

On rare occasions, the pixels per inch reported by the OS is incorrect.  In such cases, the Calibrate Tab allows you to adjust the pixels per inch value manually.  The programme will remember the value the next time it starts.  Use the arrow keys to manually change the markings on the ruler.  Horizontal arrows are enabled when the ruler is horizontal and vertical arrows are enabled when the ruler is vertical.  You may reset the pixels per inch setting at any time by clicking the reset button near the arrows.  Check out the tutorial here.