Back to GuidesBolts & Nuts
High Tensile Bolt Grade Markings: 8.8, 10.9 & 12.9 Standards Decoded
Understanding metric bolt strength class head markings. Learn how to calculate nominal tensile strength and yield stress from decimal grade numbers like 8.8 and 10.9.
The ISO 898-1 Metric Strength Marking Formula
Unlike imperial bolts that use radial slash lines to denote Grade 5 or Grade 8, metric high-tensile bolts feature two numbers separated by a decimal point stamped directly onto the bolt head (e.g. 8.8, 10.9, 12.9).
How to Calculate Tensile Strength from the Numbers
- First Number (Nominal Tensile Strength): Multiply by 100 to get minimum Ultimate Tensile Strength in N/mm² (MPa).
Example for 10.9: 10 × 100 = 1,000 N/mm² minimum tensile strength. - Second Number (Yield Ratio): Multiply the first number by the second number, then multiply by 10 to get the Yield Point (where the bolt begins to permanently deform).
Example for 10.9: 10 × 9 × 10 = 900 N/mm² yield strength (90% of tensile).
Property Class Breakdown Table
| Property Class | Tensile Strength (UTS) | Yield Strength (Rp 0.2) | Typical Material & Heat Treatment |
|---|---|---|---|
| 4.6 / 4.8 | 400 N/mm² | 240 / 320 N/mm² | Low carbon mild steel (non-structural) |
| 8.8 (High Tensile) | 800 N/mm² | 640 N/mm² | Medium carbon steel, Quenched & Tempered |
| 10.9 (Extra High) | 1,000 N/mm² | 900 N/mm² | Low-alloy boron steel, Quenched & Tempered |
| 12.9 (Ultra High) | 1,200 N/mm² | 1,080 N/mm² | High alloy chrome-moly steel (precision machine tooling) |