In most cases, when specifying the length of a headed bolt, the thickness of the bolt head is not included. For instance, if you were to order a ¾” x 24” hex head bolt, the bolt would measure 24” from under the head.
One exception to this method of measurement is countersunk bolts. When specifying the length of countersunk bolts, the head thickness is included in the overall length.
The square neck under the head of carriage bolts can cause some confusion as to how exactly the length is determined. Carriage bolt lengths are measured from under the head, not from under the square neck.
