Measuring Fastener Diameter
There are several different locations on a fastener where
one can measure the diameter.
The most commonly used diameters
- 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
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
Carriage bolt diameter is measured in the same way as hex
Lag bolt diameter is measured in the same way as hex bolts.
Thread diameter is used when measuring wood screws. 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
Only the shoulder diameter is used when measuring shoulder bolts.