How do I measure caliber/calibre?

How do I measure caliber/calibre?
 * Calibre/Caliber or cal, is the bullets diametre in inches. For example, a .50-cal BB would be 12.7mm wide. A 9mm BB is .354 inches and a 5.5mm projectile is .22 cal. (See here for a table of calibres and millimetres).