Change the unit of measurement

The default unit of measurement in Illustrator is points (a point equals .3528 millimeter). You can change the unit that Illustrator uses for general measurements, strokes, and type. You can override the default unit while entering values in boxes.

To change the default unit of measurement, choose Edit > Preferences > Units & Display Performance (Windows) or Illustrator > Preferences > Units & Display Performance (Mac OS), and then select units for the General, Stroke, and Type options. If Show Asian Options is selected in the Type preferences, you can also select a unit specifically for Asian type.
Note: The “General” measurement option affects rulers, measuring the distance between points, moving and transforming objects, setting grid and guides spacing, and creating shapes.
To set the general unit of measurement for the current document only, choose File > Document Setup, choose the unit of measure you want to use from the Units menu, and click OK.
To change the unit of measurement when entering a value in a box, follow the value by any of the following abbreviations: inch, inches, in, millimeters, millimetres, mm, Qs (one Q equals 0.25 millimeter), centimeters, centimetres, cm, points, p, pt, picas, pc, pixel, pixels, and px.
When mixing picas and points, you can enter values as XpY, where X and Y are the number of picas and points (for example, 12p6 for 12 picas, 6 points).