Understanding fastener strength property class: 4.6 vs 8.8 vs 10.9 vs 12.9
When selecting bolts for structural, mechanical or industrial use, knowing the strength grade is critical. Property classes like 4.6, 8.8, 10.9, and 12.9 refer to a fastener's tensile strength and yield strength ratio.
Using the wrong grade can lead to underperformance, failure, or overengineering (and overspending).
Interpreting the numbers
Each property class is expressed as two numbers separated by a dot:
- The first number, multiplied by 100, gives the nominal tensile strength in Megapascals (MPa)
- The second number, as a decimal, is the ratio of yield strength to tensile strength
Example:
- Property Class 8.8 = 800 MPa tensile strength, 0.8 yield ratio → 640 MPa yield strength
Property Class 4.6
Properties:
- 400 MPa tensile strength
- Low strength, low cost
Applications:
- Light-duty, non-structural
- General-purpose fittings, furniture
Property Class 8.8
Properties:
- 800 MPa tensile strength
- Medium carbon steel, heat-treated
Applications:
- Machinery, automotive components
- Structural steel connections (AS/NZS 1252.1 compliant)
Property Class 10.9
Properties:
- 1000 MPa tensile strength
- High-strength alloy steel, heat-treated
Applications:
- Heavy machinery
- Suspension systems, mining equipment
- Vibration-prone environments
Property Class 12.9
Properties:
- 1200 MPa tensile strength
- Ultra-high-strength alloy steel
Applications:
- Aerospace
- High-performance automotive
- Critical load-bearing structures
Caution: Over-specifying (e.g. using 12.9 when 8.8 will do) can:
- Increase cost unnecessarily
- Reduce ductility (more brittle under shear)
- Require stricter torque control
>> Download our head markings PDF.
Compliance and standards
Make sure your fasteners meet relevant standards:
- AS 1110.1 ISO metric hexagon bolts and screws – part 1 Bolts
- AS 1112.1 ISO metric hexagon nuts
- AS 3566.1 Self-drilling screws for the building and construction industries
- AS/NZS 1252.1 High-strength steel fastener assemblies for structural engineering - Bolts, nuts and washers
Ask for product certificates or test reports for critical applications.
Need help specifying fasteners?
United Fasteners supplies compliant bolts in all strength grades. We offer:
- Engineering-grade advice
- Batch certification
- Custom kitting and labelling
Not sure what grade you need? Talk to our technical team 1300 580 954 or see our technical fastener guides today.