Metric bolt class chart
Strength properties of metric bolt property classes, from soft 4.6 up to high-strength 12.9. The class number itself encodes the strength, so once you can read it you can size a metric bolt without a table.
Metric property classes
| Class | Proof (MPa) | Yield (MPa) | Tensile (MPa) | Typical material |
|---|---|---|---|---|
| 4.6 | 225 | 240 | 400 | Low-carbon steel |
| 4.8 | 310 | 340 | 420 | Low-carbon steel |
| 5.8 | 380 | 420 | 520 | Low-carbon steel |
| 8.8 | 600 | 660 | 800 | Medium-carbon, quenched and tempered |
| 10.9 | 830 | 940 | 1040 | Alloy steel, quenched and tempered |
| 12.9 | 970 | 1100 | 1220 | Alloy steel, quenched and tempered |
The class is two numbers separated by a dot. The first is roughly the tensile strength in hundreds of MPa, so class 8.8 is about 800 MPa tensile. Multiplying the two numbers gives roughly the yield strength in hundreds of MPa: 8 times 8 is 64, or about 640 MPa. Class 8.8 is the workhorse; 10.9 and 12.9 are high strength but more brittle.
Comparing with inch grades, or setting torque?
See the Bolt Grade Chart for SAE grades, or the Bolt Torque Calculator for tightening values.
How to read a metric class
A stamping of 8.8 on a bolt head tells you two things directly: a tensile strength near 800 MPa and a yield about 80 percent of that, near 640 MPa. The system is designed so the markings carry the strength, which is why metric bolts show numbers rather than the radial lines used on inch bolts.
Choosing a class
Class 8.8 covers most general and structural use. Step up to 10.9 or 12.9 only when the joint needs the extra preload, remembering that higher classes are stronger but more brittle and more notch-sensitive. Lower classes like 4.6 suit light, non-critical work.
FAQ
What is the strongest metric bolt class?
Among common classes, 12.9, with about 1220 MPa tensile and 1100 MPa yield. It is used for high-stress alloy-steel fasteners.
What does 8.8 mean on a bolt?
A tensile strength near 800 MPa and yield about 640 MPa. The first number times 100 is the tensile strength; the two numbers multiplied times 10 give the yield.
Is class 8.8 the same as Grade 8?
No. Class 8.8 is roughly equivalent to an inch Grade 5. The inch Grade 8 is closer to metric class 10.9.