How fixings are measured
Bolts and Screws
Generally, the length of a bolt or screw is measured from the point where the surface of the material will be when it is installed, to the end of the thread.
Therefore, bolts or screws with heads that sit on top of the surface are measured from under the head to the end of the thread, while others that sit flush with the surface are measured from the top of the head to the end of the thread.
Head Above Surface
Although there are a number of diameters on any bolt or screw, they should be measured across the widest part of the thread to save any confusion.
Thread Pitch or Threads Per Inch (tpi)
Thread Pitch is used in the measurement of metric fixings. Because threads can be quite fine, this distance is taken between the highest point between 2 threads.
Threads per inch (tpi) is simply a count of the number of threads per inch measured along the length of a fixing.