Bolt sizes are determined by the size of the threads. Measurements of bolts are in millimeters, then rounded up for ease of remembering. You can get bolts that are finely threaded, and others that are coarse. Finer threads are for more light duty applications, like paintings or household repairs. Coarse bolt threads are used in industrial applications, like trucks and machinery. A complete line
. of bolts can be seen at http://www.nutsandbolts.com/bolts-c-31.html.
