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HOW TO MEASURE 
FASTENER/SCREW LENGTHS

Reproduced by kind permission of www.boltdepot.com

 

General

Fastener length is measured from where the material surface is assumed to be, to the end of the fastener.

round headed screw

For fasteners where the head usually sits above the surface, the measurement is from directly under the head to the end of the fastener.

countersunk screw

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.

 

Hex Bolts

Hex bolts are measured from under the head to the tip of the bolt.

 

hex bolt

 

Wood Screws

Wood screw measurement depends upon the type of screw you are using.

  • Countersunk (Flat or Oval head) are measured overall.

 

flat headed wood screw          oval head wood screw

 

  • Round head screws are measured from under the head.

 

round head wood screw

 

Machine Screws

Machine screw measurement depends on the style of head it has.

  • Countersunk (Flat or Oval head) is measured overall.

 

flat head machine screw        oval head machine scew

 

  • Pan Head or Round Head screws are measured from under the head.

 

pan head machine screw       round head machine screw

 

Carriage Bolts

Carriage bolts are measured from directly under the head.

 

carriage bolt

 

Lag Bolts (Lag Screws)

Lag Bolts are measured from under the head.

 

lag bolt or lag screw

 

Sheet Metal Screws

Sheet metal screw measurement depends upon the type of screw you are using.

  • Countersunk screws (Flat or Oval head) are measured by overall length.

 

flat head sheet metal screw       oval head sheet metal screw

  • Pan Head, truss head, and Hex Head screws are measured from under the head.

 

pan head sheet metal screw       hex head sheet metal screw

 

Socket Products

Socket screw measurement depends upon the type of screw you are using.

  • Flat Socket Caps are measured by the overall length.

 

flat socket cap

  • Socket Head Caps and Button Head Caps are measured from under the head.

 

socket head cap             button head caps

 

Eye Bolts

Eye Bolts are measured from under the eye.

 

eye bolt           eye bolt

 

 eye bolt          eye bolt

 

Hanger Bolts

Hanger bolt length is measured from end to end.

hanger bolt

 

Shoulder Bolts

Shoulder bolt length is measured as the length of the shoulder.

shoulder bolt

 

Sex Bolts

Sex bolt length is measured from under the head.

sex bolt

 

 

 

This is just one of the very useful articles featured on the site of https://www.boltdepot.com others include 

Metric Tap and Drill Bit Size Table

US to Metric Conversion Table

 

 

Here's a formula sent in by Geoff Sharpe, for working out the exact diameter, in inches, of the shank of a screw. 

 

Multiply the screw Number by 13 and add ".060.

Examples -  

(No.) 8 x 13 + ".060 = ".164

(No.)10 x 13 + ".060 = ".190

 

 

 

 

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