Understanding Bolt Grades: A Complete Guide by Unifit Metalloys

What is a Bolt Grade?

A bolt grade is a designation that describes a fastener's mechanical properties, material family, and performance expectations. Bolt grade marking systems assist designers and buyers in matching the proper fastener for the intended application, including tension, shear, fatigue and environmental conditions. The head markings typically include numbers or designations like 8.8, 10.9, or A325, which correlate to the tensile and yield strength within the steels or stainless alloys. For example, tensile strength is the maximum load that a bolt can sustain while being stretched or pulled before it fails, whereas yield strength is the stress in which the bolt material will deform in a "permanent" condition.

Bolt Grades Explained- Roles of SAE, ISO, and ASTM

SAE, ISO, and ASTM are three primary organizations that publish the rules behind bolt grades. Many other bodies exist with their own documents, but these three offer clear examples for bolt grades explained and are the ones most buyers and engineers encounter.

Each system has its own naming style, yet the idea is similar—higher grade generally points to higher strength. If the grade or material is unclear, start with the head: most standards specify head markings that identify the grade or property class.

• SAE: The Society of Automotive Engineers issues inch-series specifications. SAE J429 covers bolts, screws, studs, SEMs, and U-bolts. For bolts, the J429 grade is shown by radial lines on the head, which correspond to strength requirements. This is the common reference for Grades 2, 5, and 8 in North America and is central to ASTM bolt grades comparisons.
• ISO/Metric: The International Organization for Standardization defines metric bolt grades as “property classes.” The most used document is ISO 898-1, which sets mechanical requirements for carbon and alloy steel bolts, screws, and studs and lists ten property classes (e.g., 8.8, 10.9, 12.9). Heads typically carry raised or depressed numbers showing the property class, making metric bolt grades and strength easy to identify in the field.
• ASTM: The American Society for Testing and Materials provides widely used inch-series fastener specs. ASTM A307 covers common bolts and studs and defines two grades, A and B, often marked on the head as 307A or 307B. This framework complements SAE and gives buyers a clear route for different bolt grades and their typical bolt grades and uses in general construction and industrial service.

Common Standards for Bolt Grades - Properties and Uses

Choosing the right bolt grade starts with knowing how strength, material, and markings relate to real-world loads and environments. Below is a quick look at common ASTM and metric bolt grades, what they’re made of, typical properties, and where each fits best.

• Grade 2 Bolts (Low-carbon steel): Grade 2 is made from low-carbon steel is displayed primarily as non-plated and has a propensity to corrode when exposed to harsh environments (notably moisture and rain). Standard properties of Grade 2 are approximately 60,000 psi in tensile strength; 40,000 psi yield strength; and Rockwell B hardness of 70-80. Grade 2 bolts are suited to light-duty applications, such as assembly work on furniture, simple automotive attachments, or other non-critical applications. Outdoor or high-stress applications will necessitate a higher strength, or possibly corrosion resistant grade bolt.
• Grade 5 Bolts (Medium-carbon steel): Grade 5 is typically categorized as medium-carbon steel that is usually heat treated to increase durability. In common specification practice, this will generate somewhere around 120,000 psi in tensile strength; 92,000 psi yield strength; and Rockwell B hardness of 100-120. Grade 5 bolts would more commonly be seen in day-to-day automotive, agriculture and industrial assembly where moderate strengths would suffice. Grade 5 specification bolts will deliver reliable service as fasteners in machinery, engines, and construction structurals, for applications where there are simultaneous shear and tension loads on the bolt.
• Grade 8 Bolts (High-carbon/Alloy steel): Grade 8 bolts are made from high-carbon or alloy steel and generally heat treated to increase strength to a higher level, such as around 150,000 psi tensile strength; 120,000 psi yield strength; and a Rockwell C hardness of around 33-39. Grade 8 bolts would be specified in buggy applications where heavy-duty service and performance of the vehicle or attachment is expected and would experience high amounts of torque and large clamping loads as a defined consequence of service. (for example large bolt/attachment joints in automotive, heavy machinery, or construction attachments) Grade 8 fasteners are typically selected for long service life in severe service.
• Grade 10.9 Bolts (Metric, alloy steel): Grade 10.9 refers to a metric grade of heat-treated alloy steel bolts, and typically has a tensile strength around 170,000 psi, yield strength around 136,000 psi, and a hardness value of Rockwell C 35–42. Both Grade 10.9 and Grade 12.9 are used in demanding commercial applications, such as construction, vehicle manufacturing, and heavy equipment contexts, where high clamp force and resistance to impact are needed, and it falls near the top of the metric strength-to-cost curve.
• Grade 12.9 Bolts (Metric, alloy steel): Grade 12.9 is a quenched and tempered alloy steel metric grade, with approximately 190,000 psi tensile strength, 152,000 psi yield strength, and hardness of Rockwell C 39–45. Among commercial fasteners it has one of the highest strength materials and is commonly used in aerospace, defense, precision tooling, and other applications where maximum strength, temperature resistance, and wear resistance are desired with bolt grades and strength and bolt grades and uses contexts.

Key Considerations for Bolt Grade Selection

Determining a bolt grade begins with strength, environment(s), and fit. Use this quick guide to associate ASTM bolt grades, metric bolt grades, and stainless steel bolt grades with your load, corrosion, temperature, and thread requirements.

• Load Ratings: Higher grades of bolts are made for greater load ratings and also have higher tensile strength and yield strength. The strength rating of the grade is matched to expected force on the bolt/connection/joint; for example, a Grade 5 or Grade 8 bolt may be common in ASTM bolt grades used for automotive and heavy equipment due to its impact/better clamp load ratings, where low-load rating jobs like furniture or smaller machinery would be fine for lower grade types. The idea of grade bolts and how they relate to strength helps to select the fasteners for real service loads and fatigue.
• Corrosion Resistance: Service environment is just as important as strength when considering bolt grades. For use in wet, coastal or chemically aggressive environments, the stainless steel bolt grades (A2/A4 families) or galvanized bolt grades will perform better for the long term than plain carbon steels. In a potential splash zone or chemical exposure, the bolt coatings or stainless alloys should be considered for preventing corrosion and maintaining preload, which is important in bolt grade and use across facilities and outdoor conditions.
• Temperature Resistance: When working with applications that expose materials to either continuous heat or extreme cold, it is essential to select materials that will retain their properties at the temperature they become deployed. Continuous high-temperature joints may require fasteners made from nickel alloys like Inconel or titanium alloys, whereas refrigeration and cryogenic service usually selects bolt grades of stainless steel that will retain their toughness consistent with that service. Fundamental to making a material selection based on a temperature band is that it will minimize relaxation and sustain clamp load during heavy-duty cycles.
• Threading and Size: It's essential for threads to match the nut or tapped hole exactly. You can choose from fine and coarse grade or metric screws with ISO pitch, but selecting the correct diameter, pitch and length will ensure the fastener has good engagement, sufficient torque and pre-load. Select the correct head style to avoid clearance and inappropriate load distribution. It is always prudent to check thread form and fit to avoid galling, cross threading and premature loosening in service.

Unifit Metalloys Must be your Go-to Supplier

For projects that call for the right fit between bolt grades and uses, Unifit Fastener. supplies a full range of ASTM bolt grades (Grade 2, 5, 8), high-strength metric bolt grades (8.8, 10.9, 12.9), and stainless steel bolt grades for corrosive environments. Our team maps drawings to the correct grade and finish, provides test reports with each lot, and supports quick turnarounds for construction, plant maintenance, and machinery builds, so your specification translates cleanly from print to the field.