Do you feel that no matter what you do, your units of measure seem to be wrong? Are they always set to points, for example, when you really want inches? (Sometimes centimeters are easier to work with.) Fortunately, Illustrator lets you change your mind on the fly.
To see your measurement options, choose View→Show Rulers and then right-click in Windows (Ctrl+click on the Mac) any location on the rulers at the top and side of your screen. You get a drop-down menu that shows all the units of measurement that Illustrator understands. Choose the unit you desire, and from that moment on, Illustrator uses that unit of measurement.
You can reposition rulers by clicking and dragging out from where the rulers meet at the upper-left corner. To reset them, double-click that corner.
However, you don’t have to change your unit of measurement to use a different one. In any field where you specify an amount (such as the Height and Width options in the Rectangle dialog box), just type in the amount you want, followed by the abbreviation of the unit of measurement that you want to use. If you don’t know the abbreviation, just use the whole name. Illustrator makes the conversions for you.
Want to impress your friends with a great Power User tip? You can cycle through the different measurement units by pressing Ctrl+Alt+Shift+U (z+Option+Shift+U on the Mac). It helps to have your rulers up when doing this so you can see what the current measurement system is.