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!)