How to Tell What Grade a Bolt is
Bolts are essential components in construction, automotive, and industrial applications. However, not all bolts are created equal—they vary in strength, material, and application, which is why bolt grade markings exist.
These markings indicate the bolt’s mechanical properties, including tensile strength, hardness, and material composition. Selecting the right bolt for your application ensures structural integrity and safety.
In this guide, we’ll break down bolt grade markings, how to identify them, and how to choose the right fastener for your specific needs.
Table of Contents
What Are Bolt Grade Markings?
Bolt grade markings are symbols or numbers stamped on the head of a bolt to identify its material properties, strength, and manufacturing standards. These markings are critical for selecting the right fastener for your application—especially when safety, structural integrity, and performance are on the line.
Understanding these markings helps ensure you’re using bolts that meet the required specifications for strength, durability, and compliance with industry standards.
Why Are Bolt Markings Important?
- Safety: Using the wrong bolt in a high-stress application can lead to failure.
- Load Capacity: Different bolts have varying tensile and yield strengths.
- Corrosion Resistance: Some applications require stainless steel or coated bolts.
- Standardization: Helps engineers, mechanics, and builders choose the correct fastener.
Each bolt grade follows specific standards set by governing organizations such as:
- SAE (Society of Automotive Engineers)
- ASTM (American Society for Testing and Materials)
- ISO (International Organization for Standardization)
Bolt Grades Explained and Their Markings
Bolt grades indicate how strong a bolt is based on its tensile strength, yield strength, and hardness.
SAE Bolt Grades (Imperial System)
SAE grades are used predominantly in the United States and follow a simple radial line system to denote strength.
Grade 2 Bolts
- Markings: No markings (smooth head)
- Material: Low or medium carbon steel
- Tensile Strength: 60,000 psi
- Common Uses: Light-duty applications such as furniture, woodworking, and simple household repairs.
Grade 5 Bolts
- Markings: Three radial lines on the head
- Material: Medium carbon steel, heat-treated
- Tensile Strength: 120,000 psi
- Common Uses: Automotive applications, machinery, structural components, and moderate stress environments.
Grade 8 Bolts
- Markings: Six radial lines on the head
- Material: Medium carbon alloy steel, quenched and tempered
- Tensile Strength: 150,000 psi
- Common Uses: High-stress environments such as suspension systems, heavy machinery, and military applications.
Metric Bolt Grades (ISO System)
Metric bolts use numeric markings instead of radial lines.
Class 8.8 Bolts
- Markings: “8.8” stamped on the head
- Material: Medium carbon steel, heat-treated
- Tensile Strength: 800 MPa (~116,000 psi)
- Common Uses: General construction and structural applications.
Class 10.9 Bolts
- Markings: “10.9” stamped on the head
- Material: Alloy steel, quenched and tempered
- Tensile Strength: 1,040 MPa (~150,000 psi)
- Common Uses: Automotive suspensions, heavy machinery, and industrial equipment.
Class 12.9 Bolts
- Markings: “12.9” stamped on the head
- Material: Alloy steel, quenched and tempered
- Tensile Strength: 1,220 MPa (~177,000 psi)
- Common Uses: Aerospace, robotics, and ultra-high-stress environments.
Stainless Steel Bolt Markings
Unlike carbon steel bolts, stainless steel bolts follow different grading standards:
- A2 (304 Stainless Steel): Offers corrosion resistance, commonly used in marine and outdoor applications.
- A4 (316 Stainless Steel): Higher corrosion resistance, ideal for saltwater and chemical exposure.
- F593: Covers various stainless steel fasteners in industrial and commercial applications.
Stainless steel bolts do not have the same strength as Grade 8 or Class 12.9 bolts, but they are superior in corrosion resistance.
How to Identify Bolt Grades
To determine a bolt’s grade:
- Look for head markings – Check for radial lines (SAE) or numeric codes (ISO).
- Check the material – Is it carbon steel, alloy steel, or stainless steel?
- Consider the application – High-stress environments require stronger bolts.
How to Choose the Right Bolt
Choosing the right bolt isn’t just about picking the right size—it’s about ensuring strength, safety, and reliability for your project. From material type to grade markings, each bolt is designed for specific loads and environments.
1. Application Type
- Light-duty applications (furniture, woodworking) → Grade 2 or Class 8.8
- Automotive & machinery → Grade 5 or Class 10.9
- Heavy machinery & industrial applications → Grade 8 or Class 12.9
- Marine & chemical environments → A4 Stainless Steel
2. Tensile Strength Requirements
If the bolt will bear heavy loads or high tension, a higher-grade bolt is needed.
Bolt Grades Chart:
| Bolt Grade | Tensile Strength (psi) | Common Use Case |
| Grade 2 | 60,000 psi | Light-duty |
| Grade 5 | 120,000 psi | Automotive |
| Grade 8 | 150,000 psi | Heavy machinery |
| Class 8.8 | 116,000 psi | Structural applications |
| Class 10.9 | 150,000 psi | Industrial applications |
| Class 12.9 | 177,000 psi | Aerospace |
3. Environmental Factors
- Corrosion resistance needed? → Choose stainless steel or galvanized bolts.
- Exposure to extreme heat or cold? → Alloy steel bolts perform better.
- Need for flexibility? → Lower-grade bolts have more elasticity.
Why Choose AmeriFast Supply?
Understanding bolt grade markings is essential for choosing the right fastener. The correct bolt ensures safety, durability, and optimal performance in any application.
At AmeriFast Supply, we provide a wide range of high-quality bolts and fasteners, including SAE, metric, and stainless steel options. Whether you need industrial-grade bolts or corrosion-resistant fasteners, we have you covered.
Shop bolts & fasteners today at AmeriFast Supply.
Frequently Asked Questions
1. What does the bolt head number indicate?
The number indicates the bolt’s grade:
- SAE bolts use radial lines (e.g., Grade 5 has three lines).
- Metric bolts use numeric codes (e.g., Class 10.9 means 1,040 MPa tensile strength).
2. How to tell what grade a bolt is?
Check the head markings:
- No markings → Grade 2
- Three radial lines → Grade 5
- Six radial lines → Grade 8
- “10.9” or “12.9” → Metric classification
3. Can I substitute a lower-grade bolt for a higher-grade one?
No—a lower-grade bolt may not withstand the required load, leading to failure.
4. Are stainless steel bolts stronger than carbon steel bolts?
Not necessarily. Stainless steel bolts prioritize corrosion resistance over strength. A Grade 8 bolt is stronger than an A4 stainless steel bolt.
