How can I change Prism's ruler from inches to cm, or from cm to inches?

Prism 4  or 5 or 6 (Windows or Mac)

Select between inches and centimeters on the View tab of the Preferences dialog. This changes the ruler and also the units used in several dialogs. To open the preferences dialog using Windows or Mac OS 8-9, drop the Edit menu and choose Preferences. To open the preferences dialog using Mac OSX, drop the Prism menu and choose preferences.

With Prism 5, you can also right click on a ruler and change the units.

Prism 3 Windows

There is no setting for units in Prism3 for Windows. Instead, Prism 3 reads the setting from the Windows control panel. To change the setting, go to the control panel for regional settings (international settings in older Windows versions), choose the number tab, and set the measurement system to US or Metric. Then restart Prism.

Prism 3 Mac

With Prism 3 Mac, choose units within Prism itself. Pull down the Edit menu and choose File and Printer options. Then choose inches or centimeters.