Measuring Fastener Diameter
This technical information has been supplied by boltdepot.com
There are several different locations on a fastener where one can measure the diameter. The most commonly used diameters are:
Thread Diameter (T) (also called major diameter)
Shank Diameter (S)
Root Diameter (R) (also called minor diameter)
In some cases the head size of a hex bolt (diameter across the flats) is also used. However, due to the fact that different head sizes may be used with the same diameter bolt, this is not a reliable way to determine bolt diameter. For more information on hex bolt head sizes please see our US Hex Bolt Head/Wrench Size Table or our Metric Bolt Head/Wrench Size Table.
For information on how diameters are written in bolt specifications please see our How Fastener Measurements are Notated page.
Nuts and Washers
Both nuts and washers are sized by the fastener they fit. For example a 1/2 inch nut fits a 1/2 inch bolt and a 1/2 inch washer fits the same bolt.
Note: When purchasing nuts the Thread Count or Pitch must also match that of the bolt. For more information please see our About Thread Pitch and TPI page and or How Fastener Measurements are Notated page.
The diameter of a hex bolt is the Shank diameter, expressed in inches for US bolts and Millimeters for Metric Bolts. Because this is approximately the same as the Major or Thread diameter the thread diameter measurement can be used for fully threaded bolts.
Machine screws are measured in the same way as hex bolts above.
US machine screws under 1/4 inch are then cross referenced to a size number. See our US Machine Screw Diameter Table.
Socket Head Screws
Socket head screw diameter is measured in the same way as machine screws.
Carriage bolt diameter is measured in the same way as hex bolts.
Lag bolt diameter is measured in the same way as hex bolts.
Wood screw diameter can be measured either as a shank diameter or a root diameter. The measurement is then cross referenced to a size number. See our Wood Screw Diameter Table.
Sheet Metal Screws
Thread diameter is used when measuring sheet metal screws. The measurement is then cross referenced to a size number. See our Sheet Metal Screw Diameter Table.