As a general rule of thumb, there are 25.4 millimeters per 1 inch.
To figure out how big an item is in inches, simply take the millimeter measurement and divide it by 25.4.

Example: You have a 12mm earring and you want to know the size in inches:

12 divided by 25.4= 0.47
(Which makes your earring almost 1/2 an inch!)