Measuring Fastener Diameter
General
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.
The fastener diameter is almost always the Thread Diameter (or major diameter).
Head 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 incompatible 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:
Specific Exceptions