How to Identify Nuts, Bolts, and Screws
General
Fasteners are identified by four or five attributes; Type, Material, Diameter, Length, and Thread Pitch or Count (machine thread fasteners only). This page describes what these attributes represent. Information on how they are written can be found on our How Fastener Measurements Are Notated page. Also for information on fastener abbreviations please see our Common Fastener Abbreviations page.
Note: With some fasteners, the terms bolt and screw are frequently used interchangeably.
Type
Fastener type can be divided into information on the Category or kind of fastener, Drive Type, and Head Style.
Category
Fasteners are divided into categories based on their function
or design. For help identifying different fastener categories
see our Fastener Type Chart.
Ex. Wood Screw, Sheet Metal Screw, Hex Bolt, Lag
Bolt, etc.
Drive Type
Fasteners in some categories are available with different
drive types such as Philips or Slotted. In other categories
such as lag bolts there is either only one drive type or the
drive type is implied to be of a standard type. For help identifying
different fastener drive types see our
Fastener
Type Chart
.
Ex. Philips, Slotted, Allen (a.k.a. Socket), Frearson
(similar to Philips), etc.
| Fastener Type | Standard or Implied Drive |
|---|---|
| Bolt | Hex Head |
| Lag Bolt/Screw | Hex Head |
| Carriage Bolt/Screw | No Drive |
| Socket Bolt/Screw | Allen Drive |
| Button Head | Allen Drive |
Head Style
Many categories are also available with different head shapes
or styles. For help identifying different fastener head styles
see our Fastener Type Chart.
Ex. Flat head, Pan head, Truss head, etc.
Material
Fastener material describes the material from
which the fastener was made as well as any material grade.
For help identifying different fastener materials please see
our
About Fastener Materials and
Grades
page.
Ex. Stainless Steel, Zinc Plated Steel, Silicon Bronze,
etc
Diameter
Fasteners diameter is measured either as a size number or as a direct measurement. For information on how to measure fastener diameter please see our Measuring Fastener Diameters page.
Length
How fastener length is measured varies based on the type of head. As a general rule, the length of fasteners is measured from the surface of the material, to the end of the fastener.
For fasteners where the head usually sits above the surface such as hex bolts and pan head screws, the measurement is from directly under the head to the end of the fastener.
For fasteners that are designed to be counter sunk such as flat head screws, the fastener is measured from the point on the head where the surface of the material will be, to the end of the fastener.
For more information please see our Measuring Fastener Length page.
Thread Pitch or Count
Thread pitch or thread count is used only on machine thread fasteners (those that take a nut or thread into a tapped hole). The thread pitch or count describes how fine the threads are. For more information please see our About Thread Pitch and TPI page.