Fastener Measurement
Surprisingly Simple.
Most commonly, screws and bolts are measured by diameter, thread pitch, and length. Measuring fasteners is surprisingly simple. Different tools exist to help you measure screws, nuts, washers, bolts, rivets, and other related fasteners. It is important to have the right tools and know how to measure fasteners. For example, you cannot measure a rivet with a ruler. This section was created with measuring in mind. We created this page to shed some light on fastener measuring through pictures.
How we notate fasteners.
Fastener Type, Major Diameter – Thread Pitch x Nominal Length, Material Finish
Measuring Major Diameter
The diameter of a screw or bolt is the measurement of thickness of the threaded portion.
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 because 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 inch fasteners
Measuring
Thread per Inch
TPI stands for Threads Per Inch. This is simply a count of the number of threads per inch measured along the length of a fastener. TPI is used only with Inch (Standard) fasteners.
E.g. A UNC Coarse 7/8-9NC screw has a threaded shank that is 7/8" in diameter and has 9 threads per inch. A UNF Fine 7/8-14NF screw has a threaded shank that is 7/8" in diameter and has 14 threads per inch. In general smaller fasteners have finer threads, so the thread count is higher.
For metric fasteners
Measuring
Thread Pitch
Metric fasteners are specified with a thread pitch instead of a thread count. The thread pitch is the distance between threads expressed in millimeters (measured along the length of the fastener). For example a thread pitch of 1.0 means that the distance between one thread and the next is 1.0 mm. In general smaller fasteners have finer thread so they have lower thread pitch.
Measuring Nominal Length
It is important to measure the length of a screw correctly for it to fit your application. An incorrectly measured screw can end up hindering a projects timeline. If the fastener is measured incorrectly, you may end up with fasteners that are too long or too short for your needs.
Fastener length is generally measured from the point where the surface of the material will be when the fastener is installed to the end of the fastener. Thus, fasteners with heads that sit above the surface (non-countersunk) are measured from under the head to the end of the fastener, while fasteners that sit flush with the surface (countersunk) are measured from the top of the head to the end of the fastener.
There are some 'oddball' fasteners like U bolts where length is measured differently.
Measuring Nuts
Bolts commonly use three types of measurement, while nuts use only two. Bolts are measured in length of shank (nominal length), major diameter, and the thread pitch or thread count. Nuts are measured on its inside diameter and by its thread pitch or thread count.
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.
Other Measurements
k
Head Height
l
Nominal Length
b
Thread Length
d
Major Diameter
s
Width Across Flats
All Information provided in this page serves as a guide and not for reference purposes. Citi Bolt does not guarantee or warrant the accuracy or reliability of any information in our fastener guide. Citi Bolt accepts no responsibility for any loss or damage whatsoever resulting from reliance on the information provided.