Metric bolt “classes” describe minimum strength. The first number (~tensile strength/10) and the decimal (~yield ratio) tell you a lot. Example: 10.9 → ~1000 MPa minimum tensile, ~90% yield ratio. Stainless uses a different format (e.g., A2-70 ≈ 700 MPa tensile).
| Marking | What it means | Common use |
|---|---|---|
| 12.9 | ~1220 MPa min tensile; high-strength alloy steel | Tooling, machinery, fixturing |
| 10.9 | ~1040 MPa min tensile; alloy steel | Automotive, general industrial |
| 8.8 | ~800 MPa min tensile; medium carbon steel | General assembly, frames |
| A4-80 | Stainless (≈316), ~800 MPa min tensile | Marine, corrosive environments |
| A2-70 | Stainless (≈304/18-8), ~700 MPa min tensile | General stainless hardware |
See the full chart + printable PDF
Quick FAQs
Is 10.9 the same as Grade 8?
They’re similar in minimum tensile strength, but standards and testing differ—treat as contextual equivalents only.
What does “.9” mean in 10.9?
It’s the approximate ratio for yield strength relative to tensile strength (≈90%).
2 comments
Anthony Nastus
Need a bolt that is not brittle and can with stand lateral pressures. Tensile strength not as important. Have an 8.8 JH that is braking from side to side movement.
Ravikumar Kanagaraj
hi Team,
I am looking for A4-70 and A4-80 M8 bolts for the project. Here there is no proof load is mentioned. What is the applicable tightening torque for these bolts and how it is being calculated, Please share the details