Measuring the length of a bolt or a screw
The length of a bolt or a screw is measured from the point at which the head sits flat with the material surface up until the tip of the threads. Bolts with hexagon heads are measured from under the head to the end of the threads. The same applies for socket head cap screws, button head screws, pan head screws, cheese head screws, round head screws, truss head screws, cup head square neck bolts and coach screws.
Countersunk head screws, also sometimes referred to as flat head screws, which are measured from the top of the head to the tip of the threads, in other words the entire length of the screw.
Raised countersunk head screws, also sometimes referred to as oval head screws, these are measured from the widest part of the head (very top of the countersunk part of the head) to the tips of the thread. Headless screws such as grub screws (socket set screws), threaded studs and dowel pins are measured from top to bottom, in other words the entire length.