High-Tensile Fastener Grades Explained: 8.8, 10.9 and 12.9
What the grade markings on bolts mean for tensile strength and application suitability.
The numbers stamped on a bolt head tell you how strong it is. Grades 8.8, 10.9 and 12.9 are the common high-tensile property classes for metric bolts, and understanding them prevents under-specified or over-specified fasteners.
How to read the grade
The grade has two numbers separated by a point. The first number times 100 gives the minimum tensile strength in MPa (N/mm²). The second number is the ratio of yield strength to tensile strength, as a tenth. So grade 8.8 has a minimum tensile strength of 800 MPa, and a yield of 80% of that, which is 640 MPa.
The three common grades
- 8.8: 800 MPa tensile, medium carbon steel, quenched and tempered. The standard high-tensile grade for structural and machinery bolting.
- 10.9: 1000 MPa tensile, alloy or medium carbon steel, quenched and tempered. Used where more clamp load is needed in the same size.
- 12.9: 1200 MPa tensile, alloy steel. Used for compact, heavily loaded joints, such as socket head cap screws in machine tools and tooling.
Stronger is not always better
Higher grades are harder and can be more sensitive to hydrogen embrittlement, particularly if plated carelessly, and to notches and shock. Use the lowest grade that meets the design load, and follow the designer's specification.
Match the nut
The nut must be of a matching class. A bolt of grade 8.8 is normally used with a class 8 nut, 10.9 with a class 10 nut. Mixing a high-tensile bolt with a weaker nut leaves the weaker part to fail first.
Check the markings and certificates
Genuine bolts carry the grade and the manufacturer's mark on the head. For critical joints, ask for the test certificate. Do not replace a bolt with a lower grade than the original.
Key Takeaways
- First number x 100 = tensile strength in MPa; the second sets the yield ratio.
- 8.8, 10.9 and 12.9 are the usual high-tensile grades.
- Use matching nuts and never downgrade a bolt.
This article is general guidance. For a recommendation for your own application, contact our engineering team. High-tensile fasteners →