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