How to measure screws correctly
All Screws, Bolts and Socket Caps are measured from under the head to end of thread
Countersunk are measured including the head to end of thread
Round Oval Countersunk are measured to the top of the countersunk section of head to end of thread but not including the round head
Bolts and Pan Head Screws
are measured from under the
head to the end of the thread
Round-Oval Countersunk Screws are measured to the start of, but not including the Oval
Bolt or Screw size is determined by the width of the thread
Thread length is the length of threaded section of bolt
Bolt Head Size or A/F (across flats) is the width of the bolt head (spanner size)
Bolts and Pan Head Screws with washers are measured from under the head to include the washers to the end of the thread
Countersunk Screws are measured to the top of the head
Thread Pitch is the measurement of distance between the threads
Pan Head screw head size is the width of the screw head
Nut A/F ( across flats) is the width of the nut across two flats (spanner size)