HOW TO MEASURE 
FASTENER/SCREW LENGTHS

General

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

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 head) 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 head) are measured by overall length.

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

Socket 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.

Eye Bolts

Eye Bolts are measured from under the eye.

Hanger Bolts

Hanger bolt length is measured from end to end.

Shoulder Bolts

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

Sex Bolts

Sex bolt length is measured from under the head.

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 .013 and add ".060.

Examples -  

(No.) 8 x .013 + ".060 = ".164

(No.)10 x .013 + ".060 = ".190

 

My thanks to Jason Lee Olson for correcting the original formula shown

Jason has also suggested you could convert the screw number to to decimal 
e.g. No. 8 = .008 and use the following formula.

Screw No. 8 - .008 x 13 + .060 = .164 (No. 8 diameter in inches)

Screw No. 6 - .006 x 13 + .060 = .138 (No. 6 diameter in inches) 

 

Should you require the diameter in mm then multiply the answer by 25.4.

Screw No. 8 - .008 x 13 + .060 = .164 (No. 8 diameter in inches) x 25.4 = 4.166 diameter in mm

Screw No. 6 - .006 x 13 + .060 = .138 (No. 6 diameter in inches) x 25.4 = 3.505 diameter in mm

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