From basic hardware to structural-grade: how the most common low-strength and high-strength metric classes compare.
Proof stress: 225 MPa · Tensile: 400 MPa
No heat treatment required
Use: Non-structural, light fixings
Proof stress: 580 MPa · Tensile: 800 MPa
Quenched & tempered
Use: Structural, automotive, machinery
Class 8.8 bolts have 2.6× the proof stress of class 4.6. This is the most significant grade jump in the ISO system — the difference between a bolt you'd find in flat-pack furniture and one holding an engine together.
Class 4.6 is perfectly adequate for non-load-bearing applications: cable trays, light brackets, enclosure covers, and furniture. But any application with dynamic loading, vibration, or structural responsibility needs at minimum class 8.8.
The cost difference is modest — class 8.8 typically costs 30–50% more than 4.6 in the same size. Given the massive strength improvement, there's rarely a good reason to use 4.6 where 8.8 would be safer.