Bolt Strength Grades

Metric property classes

The two-number class (e.g. 8.8, 10.9) is a designation built from the bolt's nominal strength: the first number × 100 gives the nominal tensile strength in MPa, and the second number relates to the yield-strength ratio. The actual specified minimum tensile strength (below) can run slightly above this nominal figure and can also vary by diameter within a class.

SAE grades

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, minimum tensile strength 1220 MPa). 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.