Call Us Today! (+86) 188-2253-7569 |

An Ultimate Guide to American Fastener Sizes

If you need the definitive guide to American fastener sizes, you’ve arrived at the perfect spot! Fasteners are an essential tool for any DIYer, professional contractor, or engineer, and the right size and type can make the difference between a successful project and a failed one.

In this article, we’ll take a look at the different types of fasteners and the American fastener size chart, as well as the different grades, coatings, and other factors to consider when choosing fasteners. Let’s get started!

fastener design

Introduction of Fastener Types

Fasteners come in a variety of types, including screws, nuts and bolts, rivets, and washers.

  1. Screws are the go-to fastening solution for a wide range of tasks. They can be used in wooden, metal, and plastic surfaces, as well as other materials.
  2. Nuts and bolts are also popular fasteners and come in many sizes and materials. They are frequently used to join two metal components and come in various thread measurements.
  3. Rivets are also used to join two pieces of material together and come in different sizes, shapes, and materials.
  4. Finally, washers are used to protect the surface of a fastener and are available in different materials.

All of these fastener types are essential to any manufacturing process and help ensure the materials are secured together properly.

Understanding the American Fastener System

American fastener sizes refer to the standardized sizing system used to describe the size, length, and thread pitch of fasteners used in the United States. This system is based on the National Aerospace Standard (NAS). This standard was developed by the aerospace industry to ensure consistency in the design and manufacture of fasteners.

The NAS is divided into two categories: inch-based and metric-based fasteners.

Inch-based fasteners use imperial measurements, while metric-based fasteners use metric sizes.

Fasteners come in various sizes and types, and understanding the American fastener size chart is essential for ensuring you get the right size for your project.

How To Determine A Fastener Size?

The size of a fastener is determined by its diameter, the number of threads per inch (TPI), and length.

The diameter is the span from one edge of the thread to the other, while the threads-per-inch (TPI) denotes how many threads are within a linear inch.

Generally, the length of the bolt is gauged in inches or millimeters, determined by measuring from the crown of the head right down to the end.

Understanding Thread Pitch and Thread Count

To appropriately identify the size of a fastener, it’s vital to understand thread pitch and count.

When measuring threads, thread pitch denotes the space between each thread, while thread count indicates the number of threads present per inch.

The thread pitch and thread count will determine the size of the fastener.

For example, a #4 fastener has a diameter of 0.112 inches and 16 threads per inch (TPI). This means the thread pitch is 0.072 inches and the thread count is 16. On the other hand, a #10 fastener has a diameter of 0.190 inches and 24 threads per inch (TPI). This means the thread pitch is 0.080 inches and the thread count is 24.

The Difference Between Imperial and Metric Fastener Sizes

  • Understanding the difference between imperial and metric fastener sizes will help you choose the right size for your project.
  • Imperial fastener sizes refer to the diameter and length of the threaded portion of a fastener, measured in inches. Metric screw sizes refer to the diameter and the length of the threaded portion of the fastener, measured in millimeters.
  • Imperial fastener sizes are typically used for small fasteners, while metric fastener sizes are used for larger fasteners.

Understanding Head Types and Drive Types

There’s a range of head and drive styles when it comes to fasteners.

Head Types

The five most commonly seen head shapes are Flat, Pan, Truss, Oval, and Hex.

screws head

  • A flat-head fastener is the most common type; it has a flat top with a tapered underside for easy installation.
  • A pan-head fastener has a low profile with a wide but slightly domed top. It’s often used in applications where a low profile is desired.
  • The truss head is larger and has a rounded top. It’s often used in heavier-duty applications.
  • The oval head fastener has a low profile and an oval shape. It’s often used in applications where a low profile is desired.
  • The hex head fastener is the most common type; it has a hexagonal shape and is often used in high-torque applications.

Drive Types

Five of the most frequently encountered Drive types are Phillips, Robertson, Torx, and Hex.

screws inserts

  • The Phillips drive is the most common in carpentry and woodworking applications.
  • The Robertson drive type is great for heavy-duty applications, as it’s less likely to strip out than Phillips.
  • The Torx drive type is popular in electronics, as its star-shaped recess is highly resistant to cam-out.
  • And finally, the hex drive type is great for applications that require extra torque, as its hexagonal shape allows for more surface contact with the fastener.

So the next time you choose a drive type for a project, you’ll know which one is best for the job!

American Fastener Sizes

There isn’t an “ultimate guide” to American fastener size because there is no definitive authority. Still, a few things are vital to keeping in mind when it comes to sizing fasteners.

Unified Inch Screw Threads (UNC/UNRC) Vs. National Pipe Thread (NPT)

Firstly, it’s vital to know that there are two main types of measurement systems used for fasteners in the US – Unified Inch Screw Threads (UNC/UNRC) and the National Pipe Thread (NPT). The UNC/UNRC system is more commonly used for small-diameter screws and bolts, while NPT is typically used for pipes and fittings.

Methods For Measuring TPI

There are two ways of measuring threads per inch- which can make things challenging if you don’t know what you’re doing.

The first method is called “counting threads,” where you simply count the number of threads within a one-inch span.

The second method is called “measuring threads,” which uses a tool to measure the distance between adjacent threads.


  • Generally speaking, UNC/UNRC sizes are expressed in terms of diameter and thread count (e.g., 1/4-20), while NPT sizes are expressed as diameter + TPI (e.g., 1/4 NPT). With this information in mind, here are a few tips for sizing American fasteners:
  • For UNC/UNRC screws and bolts, start by determining the desired diameter (using fractions or decimals), then choose a thread count based on how coarse or fine you want the threads to be. Coarse threads are better for thicker materials, while fine threads are better for thinner materials.
  • For NPT pipes and fittings, start by determining the desired diameter (again using fractions or decimals), then choose an appropriate TPI based on how tight or loose you want the connection to be.
  • Lower TPIs will result in a tighter connection, while higher TPIs will be looser. When in doubt, it’s always best to err on the side of caution and go with a slightly larger size rather than a smaller one – it’s much easier to make a big hole smaller than it is to make a small hole bigger!

US Bolt Dimensions

When it comes to bolts, the US has specific dimensions that must be followed. It’s imperative to be aware of the dimensions of the bolt you’re using to guarantee a precise fitting and proper performance.

  • The diameter of the bolt is measured from the outside of the threads, and the length is measured from the underside of the head to the end of the bolt. When measuring a metric bolt, the diameter is measured across the bolt’s major diameter, and the length is measured from the top of the head to the end of the bolt.
  • In addition to diameter and length, the US also has specific thread pitch requirements that must be followed. All US bolts must have a thread pitch of 8 threads per inch, which means 8 threads are measured over an inch of the bolt.
  • It’s also vital to know the full range of sizes available. Smaller bolts, for instance, can range from 0-80 to 1/4 inch in diameter. On the other end of the size spectrum, larger bolts can measure up to 1-1/2 inches in diameter. Likewise, bolt lengths can range from 1/8 inch to 20 inches, depending on the application.
  • It’s essential to bear in mind the kind of threads a bolt has, which is usually determined by the diameter and TPI. For instance, a 1/4-inch diameter bolt may have 20 threads per inch.

When choosing the right bolt for the job, it’s paramount to ensure the size and thread type are compatible with the materials being joined together.

American Screw Sizes

Understanding the different screw sizes available in the United States is crucial for any project. The most common sizes for American screws are #4, #6, #8, and #10.

  • #4 screws are 0.112 inches in diameter and 16 threads per inch (TPI).
  • #6 screws are 0.138 inches in diameter and 18 threads per inch (TPI).

American Nut Sizes

American nuts can be found in various shapes and sizes; ranging from #2 to #12. The size is dependent on the diameter, thread pitch and length of the nut.

The most common sizes for American nuts are #6, #8, #10, and #12.

Different Fastener Materials

Fasteners come in a variety of materials, each with its properties and applications. The most frequent materials include:

  • Steel
  • Stainless Steel
  • Brass
  • Aluminum

For example, Steel is the most common material used for general-purpose applications, and Stainless Steel is corrosion-resistant and is often used in marine applications.

Fastener Grades

Fasteners come in a range of grades, each providing its own benefits for various types of work; e.g, grades 2, 5, 8, and 10 are common.

Grade 2 fasteners are low-strength and are often used in light-duty applications. Grade 5 fasteners are medium-strength and are often used in general-purpose applications. Grade 8 fasteners are often used in heavy-duty applications.


Which Of These Has Extra Strength? 10-32 Vs 1/4-20

While it is true that the 10-32 screw has finer threads and thus can create a stronger connection, it is also much smaller in diameter than the 1/4-20 screw. It means the 1/4-20 screw can handle more weight and force before breaking. So, if you need a strong connection, go with the 1/4-20 screw. If you need a finer connection (for example, working with delicate materials), go with the 10-32 screw.

What Does M8 1.25 Mean?

M8 1.25 is a metric screw designation. The “M” denotes that it is a metric screw, while the “8” specifies the nominal outer diameter of the screw in millimeters. The “1.25” specifies the pitch, or distance between threads, in millimeters.
This particular screw is thus 8mm in diameter with 1.25mm between threads.

What Is The Size 9 Fastener Diameter?

Size 9 fasteners are some of the larger screws and bolts you can purchase. Their diameters range from 0.375 to 0.500 inches, which is significantly larger than most other fasteners. This size is typically used for heavy-duty applications, such as attaching metal plates or securing large pieces of wood.

How To Quickly Choose The Right Size Fastener?

Here are some tips on how you can choose the right size screw or bolt depending on what type of job you’re doing.

  • First, identify the material thickness of what you’re attaching the fastener to.
  • Second, determine how much weight will be applied to the joint.
  • Lastly, decide on the level of shear or tension strength required.


In conclusion, understanding American fastener sizes are essential for any DIYer, professional contractor, or engineer. The American fastener sizes guide provides information on the various types of fasteners available in the United States and their sizes. It includes both standard and metric measurements. This guide is intended for engineers, machinists, and other professionals who work with fasteners regularly. 


Leave a Reply

Related Posts

CNC machining factory
The Reasons to Choose Machined Parts over Molded
cnc machining workshop
What is cnc material used for?
Precision CNC Turned Part Samples
What Materials Can CNC Turning Companies Work With?
Aluminum Products One Stop Machining Service
2024 Top 10 Aluminum Fabrication Factories in China
Injection Moulding Process
An Ultimate Guide to Injection Moulding
2024 Top 10 Injection Molding Companies in China

Contact Us