How Fastener Measurements are Notated
Basic Identification
Below is an example of a full fastener description. This notation includes all of the information needed to identify the fastener.
| Fastener description: | Machine screw | Phillips pan | Stainless steel 18-8 | 1/4 - 20 x 2" | |
| Fastener type | Head | Material | Size |
Fastener Type
Fastener Type is the general type of fastener, such as wood screws, hex bolts, machine screws, hex nuts or carriage bolts.
Head
Head types contain up to two parts:
| Phillips | Pan |
| Drive type | Head style |
Drive Type
Drive type describes the type of tool used to install the fastener. Common examples are phillips, slotted, and square drives.
Some fasteners, such as carriage bolts, do not have a drive and therefore no drive type is specified.
In certain other cases, such as with hex bolts, the head and drive type (hexagonal) is implied by the fastener type.
Head Style
Head style describes the shape of the head. Common examples are pan, flat, truss, and hex.
A few fastener types, including set screws and some anchors, do not have a head and the head property will therefore not be present.
Material
The most common parts of a material description are:
| Example: | Zinc plated | Grade 8 | Steel |
| Plating | Grade | Material |
Plating
Many fasteners, especially steel fasteners, are plated or coated for corrosion resistance or decorative purposes. Common platings include zinc plating, galvanizing, and chrome plating.
Grade
Some materials, such as steel, come in various grades. The grade specifies an exact set of mechanical properties. Examples of common steel grades include grade 2, grade 8, and class 8.8.
Material
This is the basic underlying material. The most common fastener material is steel (including stainless steels), often further specified with a grade (grade 8, etc.). However, many other materials are used including, brass, bronze, and nylon.
This property will always be present even if no grade or plating is specified. Thus, a full material description for a fastener might simply be: Brass.
Other Information
Occasionally the material description will contain other information. Examples include fasteners with painted heads, colored platings such as yellow zinc, or polished finishes.
For more information on materials see our Materials page.
Size
For most fasteners, the size consists of two or three parts. For example:
| Example: | 1/4" | - 20 | x 3" |
| Diameter | Thread count |
Length |
Diameter
Diameter is typically measured on the outside of the threads. For US fasteners this is measured in inches (except for small diameters, where diameters are numbered), and for metric fasteners it is measured in millimeters (abbreviated mm or prefixed by M).
For more information on how to measure the diameter of specific fastener types see our Measuring Fastener Diameter page.
Thread Count/Pitch
Only machine threaded fasteners (nuts, and screws/bolts that could take a nut) specify a thread count or thread pitch.
US fasteners specify threads per inch (TPI), commonly called thread count, so 20 would represent 20 threads per inch. Metric fasteners instead specify a thread pitch which is the distance between the threads. Therefore, a 1.5 pitch would have 1.5 millimeters between each thread.
For more information see our Thread Pitch and Thread Count page.
Length
Fastener length is usually measured from where the surface of the material is presumed to be when the fastener is installed, to the end of the fastener. US fasteners are measured in inches, while metric fasteners are measured in millimeters (mm). For more information on how to measure specific fastener types see our Measuring Fastener Length page.
Order and Symbols
Diameter, Thread count/pitch, and Length should always be specified in this order. In addition, slightly different notation is used for US fasteners and Metric fasteners.
In US fasteners, a dash should be used to separate the diameter and thread count (if there is a thread count), while an x is used to seperate them from the length. A double quote (") may or may not be present to indicate the measurement is in inches. A number sign (#) indicates a numeric diameter used with smaller screws. Dropping the number sign for these sizes should be avoided as it can easily result in confusion between US and Metric sizes.
In Metric fasteners, an x is used to seperate each of the parts of the size. Each part (including the thread pitch) is a measurement in millimeters, so each may be followed by the abbreviation mm. Often this is left off from the thread pitch. Sometimes it is also left off of the other parts of the size. This should be avoided as it can lead to confusion with US fastener sizes. To shorten metric sizes many people use a capital M in front of the diameter, and then leave the units off of the other parts of the size. This method results in a shortened size that is still clearly a metric size.
Examples:
|
US Machine thread
US Non-machine thread |
Metric machine thread
Metric non-machine thread
Metric alternative |
Other Properties
Some fasteners have additional special properties. Some examples are special point types (thread cutting, piercing, dog point), integrated washers (neoprene sealing washers, fixed lock washers), special thread locking systems (nylon patch, pre-applied thread locker), and vented screws. These properties are included with the rest of the identification.
Nuts and Washers
Nuts and washers lack many of the properties of other fasteners.
Nuts and washer sizes are the same as the diameter of the fastener they are meant to work with. Thus, a 1/4" washer fits a 1/4" bolt/screw.
Example of a washer description:
| Example: | Flat washer | Stainless steel | 1/4" |
| Type | Material | Size |
Example of a nut description:
| Example: | Hex Nut | Stainless steel | 1/4" - 20 |
| Type | Material | Size |
Types, materials, and sizes are specified as above with the noted exceptions.
Order of Properties
While Bolt Depot uses the order seen at the top of this page, other suppliers may use a different order for the parts that make up the description.
Example:
Instead of
Type Head type Material Size
You might see
Material Type Size Head type
or
Size Material Head type Type
In other cases these various elements may be separated on a label or ordering sheet. As long as all elements are present the fastener can be easily identified.
Abbreviations
Because fastener descriptions can become quite long, abbreviations are often used.
Examples:
WS = Wood Screw
MS = Machine Screw
Phil = Phillips
S/S = Stainless Steel
G8 = Grade 8 Steel
Thus you may see something like this:
Despite being greatly shortened this contains the full fastener specification.
Many common abbreviations can be found on our Fastener Abbreviations page.
Note: In addition to abbreviations many people will leave out parts of the fastener description that they expect to either be the 'standard' or that they do not care about. For example, leaving off the thread density because they just want 'standard' (coarse) thread, or not specifying a material grade. It is always better to try to obtain this information prior to making a purchase to avoid errors.
Shop Talk
Our Printable Fastener Type Chart is a comprehensive illustrated guide to fastener types, drive types, head styles and more.
Everyone who works with fasteners eventually starts using their own abbreviations and terminology. Shouting "Grab me some railing anchors" is a lot easier than "Grab me some three eighths sixteen by four inch stainless steel stud anchors". Often this 'Shop Talk' gets handed down to people who never knew another name for the fastener and sometimes even becomes industry or regional slang.
For those times when you can't identify a fastener by name, we have created a Printable Fastener Type Chart. This type chart, in addition illustrations in our catalog, are designed to help guide you through identifying what you need right down to the specific size.
For help locating a fastener please feel free to contact our customer service department at 866-337-9888