How is the Length of a Bolt or Set Screw Measured?

The general rule for measuring bolts and set screws

For almost all types of bolts and set screws, the rule is that you measure from the underside of the head to the end of the thread, as illustrated below;

MEASURING BOLTS

The rule for measuring countersunk bolts and set screws

For countersunk bolts and socket screws, you measure the total length of the screw including the head.

MEASURING SET SCREWS

One other exception to these rules is for a less common type of fixing - the socket shoulder screw. Socket shoulder screws are only measured from under the head to the end of the plain shoulder, not including the thread.