Fastener Grades Explained: SAE, ASTM, and ISO Standards

Fasteners can be either a key component that ensures a project's success—or the reason it fails. Each fastener serves a specific purpose and is designed for a particular application. After all, you wouldn’t use the same fastener to hang a picture frame as you would to secure heavy construction equipment.
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To make these differences clear, an entire grading system exists. This system classifies fasteners based on factors like material composition, tensile strength, and intended use. Understanding these grades helps ensure you're choosing the right fastener for the job, whether it's a simple household task or a demanding industrial application.

​There are three main, commonly used grading systems:

• SAE (Society of Automotive Engineers)
• ASTM (American Society for Testing and Materials)
• ISO (International Organization for Standardization)

The choice of system depends on the country and the type of application. We will delve deeper into each of them.

1. The first, and most commonly used in the US, system is SAE, it was created by the Society of Automotive Engineers and is used to classify various products, including bolts, steel, and lubricating oils.

• ​​Grade 2:

This grade is commonly made from low or medium carbon steel and has a relatively low tensile strength of approximately 60,000 psi. It is primarily used in non-critical applications where high strength is not required. Common uses include light-duty household repairs (such as furniture and shelving) and low-stress components in HVAC systems.
How to Identify It:
Grade 2 bolts usually have no head markings (on U.S. standard bolts). Their lack of identifying lines or symbols is a key visual indicator that they are not heat-treated and are intended for low-strength applications.

• Grade 5:

This grade is made from medium carbon steel and is heat-treated to increase its strength. It has a medium tensile strength of approximately 120,000 psi. Grade 5 fasteners are commonly used in applications that require more strength than Grade 2 but not the maximum available. Typical uses include automotive components, agricultural machinery, and industrial equipment.
How to Identify It:
Grade 5 bolts can be identified by three radial lines on the head. These markings indicate that the bolt has been heat-treated and meets the strength requirements for this grade.

• ​​Grade 8:

Grade 8 fasteners are made from medium carbon alloy steel and are quenched and tempered for maximum strength. They have a high tensile strength of approximately 150,000 psi, making them suitable for high-stress applications. Common uses include heavy machinery, military vehicles, and critical automotive parts such as suspension or drivetrain components.
How to Identify It:
Grade 8 bolts have six radial lines on the head. These markings are a clear indication of their high strength and heat-treated condition, distinguishing them from lower-grade fasteners.

2. The next one is ASTM (by American Society for Testing and Materials) fasteners, which are commonly used in the construction, structural, and civil engineering industries. The grading focuses more on application and use rather than general strength classification.

• A307:

Made from low carbon steel with low tensile strength (~60,000 psi). Commonly used in light-duty structural applications, such as pipe supports, and general hardware.
How to Identify It:
Marked as “307A” on the bolt head. It typically has a plain finish, with minimal head markings beyond the grade and manufacturer code.

• A325:

Used in structural steel connections, such as steel buildings, bridges, and towers. Made from medium carbon steel and heat-treated to provide higher strength (~120,000 psi).
How to Identify It:
Head markings vary by manufacturer but usually include “A325” along with specific code symbols.

• A490:

Even stronger than A325, these bolts are used in critical structural connections requiring high strength, such as heavy crane supports, and high-rise buildings. They have a tensile strength of 150,000 psi or more.
How to Identify It:
Clearly marked with “A490” on the head. These bolts may also have restrictions such as no galvanization due to hydrogen embrittlement risks.

3. The last one on our list is ISO Standard. Metric fasteners follow standards set by the International Organization for Standardization, the most used standard in the world, especially in Europe and Asia. Metric fasteners use a two-number system (like 8.8 or 10.9).
The first number indicates tensile strength in MPa divided by 100 (e.g., 8 = 800 MPa), and the second number is the ratio of yield strength to tensile strength (e.g., 0.8 = 80%).

• 8.8:

This is a medium-strength, heat-treated fastener made from medium carbon steel. It has a tensile strength of about 800 MPa and is commonly used in automotive, manufacturing, and general machinery. Can be compared to SAE grade 5.
How to Identify It:
Look for the numbers “8.8” stamped on the bolt head. These are typically either raised or indented, depending on the manufacturer.

• 10.9:

Made from alloy steel and heat-treated, this bolt has a tensile strength of around 1,000 MPa. It's used in high-stress industrial equipment, vehicle suspension systems, and engine assembly.
How to Identify It:
Stamped with “10.9” on the head. It may also have manufacturer marks depending on the standard followed.

While SAE, ISO, and ASTM fastener grades may look similar at a glance, they serve different purposes and are designed around distinct standards and applications:

• SAE grades are best suited for automotive and machinery applications in North America, offering a simple strength-based grading system.
• ISO grades use the metric system and are prevalent in global manufacturing, especially in countries where metric sizing is standard.
• ASTM grades are engineered for structural and construction projects, emphasizing application-specific performance in infrastructure.

Choosing the right fastener grade isn't just about size or appearance — it's about matching the fastener's material properties, strength, and purpose to the specific demands of your project.