Measuring Fastener Length
This technical information has been supplied by boltdepot.com
General
Fastener length is measured from where the material surface is assumed to be, to the end of the fastener.
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For fasteners where the head usually sits above the surface the measurement is from directly under the head to the end of the fastener.
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For fasteners that are designed to be countersunk, the measurement is made from the point on the head where the surface of the material is, to the end of the fastener.
Specific Types
Hex Bolts
Hex bolts are measured from under the head to the tip of the bolt.
Wood Screws
Wood screw measurement depends upon the type of screw you are using.
Countersunk (Flat or Oval head) are measured overall.
Round head screws are measured from under the head.
Machine Screws
Machine screw measurement depends on the style of head it has.
Countersunk (Flat or Oval) is measured overall.
Pan Head or Round Head screws are measured from under the head.
Carriage Bolts
​Carriage bolts are measured from directly under the head.
Lag Bolts (Lag Screws)
Lag Bolts are measured from under the head
Sheet Metal Screws
Sheet metal screw measurement depends upon the type of screw you are using.
Countersunk screws (Flat or Oval) are measured by overall length.
Pan Head, truss head, and Hex Head screws are measured from under the head.
Socket Head Products
Socket screw measurement depends upon the type of screw you are using.
Flat Socket Caps are measured by the overall length.​
Socket Head Caps and Button Head Caps are measured from under the head.
