Metric Bolt Grade Markings & Steel Strength Chart (with US Equivalents)
This guide explains common metric bolt classes (12.9, 10.9, 8.8, A4-80, A2-70), what their head markings mean, and how their proof load, yield strength, and tensile strength compare. We also include a US/SAE overview for context—useful for mixed benches—plus a printable chart.
Download: Printable Metric Bolt Grade Chart (PDF)
Key terms (quick definitions)
- Proof load: the maximum axial load a bolt can withstand without permanent set.
- Yield strength: stress at which permanent deformation begins.
- Tensile strength: minimum ultimate stress before fracture (failure in tension).
- Conversion: 1 MPa = 1 N/mm² ≈ 145 PSI (multiply MPa by 145 to estimate PSI).
Metric Bolt Classes & Mechanical Properties
Values below reflect widely used minimums for common classes. Always verify against your specific standard and drawing.
| Head Marking | Metric Class | Proof Load (MPa) |
Min Yield (MPa) |
Min Tensile (MPa) |
Typical Material / Notes |
|---|---|---|---|---|---|
 |
12.9 | 970 | 1,100 | 1,220 | Alloy steel, quenched & tempered. |
 |
10.9 | 830 | 940 | 1,040 | Alloy steel, quenched & tempered. |
 |
8.8 | 580–600 | 640–660 | 800–830 | Medium carbon steel, quenched & tempered. |
 |
A4-80 (SS) | N/A | 600 | 800 | Stainless (≈316) for marine/corrosive environments. |
 |
A2-70 (SS) | N/A | 450 | 700 | Stainless (≈304/18-8). Most common stainless class. |
About “equivalents” (US vs Metric)
US/SAE grades (e.g., Grade 8, Grade 5, Grade 2) and metric classes use different systems and test methods. Numbers can be similar but aren’t strict 1:1 equivalents. For mixed benches, these typical minimum tensile strengths are often cited for context:
| US / SAE Grade | Min Tensile (PSI) | Approx (MPa) | Closest Metric Class (context only) |
|---|---|---|---|
| Grade 8 | 150,000 | ≈ 1,034 | ~10.9 (context) |
| Grade 5 | 120,000 | ≈ 827 | ~8.8 (context) |
| Grade 2 | 74,000 | ≈ 510 | ~5.8 / 4.6 (context) |
Practical notes
- Don’t mix systems: inch vs metric threads aren’t interchangeable—even if tool sizes seem close.
- Stainless & galling: use anti-seize and proper speed; see our guide on thread galling prevention.
- Torque: always follow the drawing and spec; see socket screw torque information.
- Head markings: verify markings match the class/grade; don’t assume by finish alone.
Related charts & tools: