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.
The fastener diameter is almost always the Thread Diameter (or major diameter).
It is common for people to refer to hex bolts by the size of the head measured across the flats (this is also the size wrench the bolt uses).
This is incorrect and should be avoided for two reasons.
- Miscommunication can result in getting a much larger bolt than needed.
- Head size can vary for the same thread diameter, especially in metric bolts, so even comparing heads to heads you may get an incompatable bolt.
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.
Nuts and Washers
Both nuts and washers are sized by the fastener they fit.
For example a 1/2 inch washer fits a 1/2 inch bolt. With a nut the thread density must also match. For more information on thread density please see our About Thread Pitch and
TPI page or How Fastener Measurements
are Notated page
A 1/2" washer fits a 1/2" diameter fastener
A 1/2"-13 nut fits a 1/2"-13 fastener
US Numbered Screw Sizes
US fasteners below 1/4" are typically referred to by a numbered size (often preceded by a number sign) rather than a fractional size. The smaller the number the smaller the diameter of the fastener.
For information on what these sizes translate to inches see our:
Some fasteners such as shoulder bolts, and many anchors use a different mesurement for diameter. Throughout our catalog you will find images showing exactly where the diameter is measured on each fastener when you are choosing a diameter.