As a general rule, thread length is matched to the head configuration. For flathead or countersunk bolts, the overall length of the bolt is from the top of the head to the end of the bolt. The actual thread length is not specified. For heads that stick up from the mounting surface, such as a hex head, the actual bolt length is measured from the bottom of the head to the end of the bolt. The overall premise is to measure from the top of the surface that the bolt is mounted on to the end of the bolt.
A shank is the area below the head that has no thread. For many applications, you want the threads to catch, but not the entire bolt length. For example if you are bolting together two plates of steel, you only want the bolt to grip the second plate, but not the first plate riding next to the head. The actual thread length is only the length of the thread, without measuring the length of the shank.
A difference exists between what defines a bolt and what is a screw. No clear-cut line exists between the two. Screws, as a rule of thumb, refer to a fastener that has only a few threads per inch, like a wood screw. A bolt has finer, or more threads per inch, than a screw. Some machine bolts are referred to as screws, such as a machine screw. Jargon and slang permeate the nut and bolt industry, so discerning between the two is ambiguous. If you refer to a wood fastener with coarse threads as a screw, and a fastener with fine threads as a bolt, you are using the correct jargon. Defining the thread length on a wood screw is tricky. Many screws have shanks, but many bolt shops usually measure the entire length of the bolt, shank included.
For metric bolts, the length of the thread is interrelated with the diameter and the length in standard sizes. For example, for a bolt that has a diameter of 10 mm and a length of 125 mm or less, the length of the thread is 26 mm. For a 10 mm diameter bolt that is 126 mm long or longer, the thread length is 32 mm. Some specially designed metric bolts may be non-standard, and have differing thread lengths from the norm.