Bolt Strength Grades
Metric property classes and SAE grades define bolt strength. Choosing the right grade ensures the joint can carry the load without failure or unnecessary cost.
Metric property classes
The two-number class (e.g. 8.8, 10.9) indicates tensile strength and yield ratio. First number × 100 ≈ minimum tensile strength in MPa; the second number relates to yield strength ratio.
| Class | Tensile (min. approx.) | Typical use |
|---|---|---|
| 8.8 | 800 MPa | General purpose |
| 10.9 | 1000 MPa | High strength |
| 12.9 | 1200 MPa | Alloy, highest |
SAE grades
| Grade | Head marking | Use |
|---|---|---|
| Grade 2 | None or minimal | Low stress |
| Grade 5 | 3 radial lines | General / automotive |
| Grade 8 | 6 radial lines | High strength |
See Bolt Head Markings for how to read them.
Torque by grade
Higher grades allow higher clamp load for the same size. Use our Bolt Torque Calculator and select the correct bolt size and grade for recommended torque (dry vs oiled). Never exceed the specification for the application.
FAQ
What is the strongest metric bolt grade?
12.9 is the highest common metric property class (alloy, ~1200 MPa tensile). For critical applications always verify the specification and use correct torque.
Is Grade 8 stronger than Grade 5?
Yes. Grade 8 has higher tensile and yield strength than Grade 5. Use Grade 8 for high-stress applications; Grade 5 is common for general and automotive use.
Does bolt grade affect torque?
Yes. Higher-strength bolts can be tightened to higher torque for the same size. Use our Bolt Torque Calculator and select the correct grade for safe values.
Related
Tools: Bolt Torque Calculator, Metric to Imperial Converter
Charts: Universal Screw & Bolt Size Chart, Screw Size Chart
Guide: Bolt Head Markings