Bolt Strength Grades
SAE and metric bolt strength grades—what they mean and how to choose the right grade. Use our torque calculator for safe tightening.
What bolt grade means
Bolt grade (SAE) or property class (metric) defines minimum tensile and yield strength. Higher grade = stronger bolt and usually higher recommended torque. Never substitute a lower grade where a higher one is specified. Use our bolt torque calculator and select the correct grade for safe clamp load. See bolt head markings to identify grade on the fastener.
Bolt strength grade chart
| SAE grade | Metric class | Approx. tensile (MPa) | Typical use |
|---|---|---|---|
| Grade 2 | 4.6 | ~400 | General purpose |
| Grade 5 | 8.8 | ~830 | Automotive, machinery |
| Grade 8 | 10.9 | ~1040 | High strength, suspension |
| — | 12.9 | ~1220 | Highest common metric |
Grade 5 vs Grade 8
Grade 5 is medium carbon; Grade 8 is alloy steel and stronger. Use Grade 8 where specs require it (e.g. suspension, structural). Don’t over-torque Grade 5 bolts in the hope of matching Grade 8—you risk stripping or breaking. Match fastener to design and use our screw size chart and tap drill chart for sizing.
FAQ
What does bolt grade mean?
Grade indicates minimum tensile and yield strength. SAE (Grade 2, 5, 8) and metric (8.8, 10.9, 12.9) define safe load capacity.
Is Grade 8 stronger than Grade 5?
Yes. Grade 8 has higher tensile and yield strength. Use Grade 8 for high-stress applications; Grade 5 for general use.
Are metric bolts stronger than SAE?
Strength depends on grade/class, not system. Metric 10.9 ≈ Grade 8; 8.8 ≈ Grade 5. Use our torque calculator for the correct grade.