What do the grade markings on metric bolts indicate?

Metric bolt grades like 8.8, 10.9, 12.9 indicate mechanical properties. The first number times 100 gives tensile strength in MPa (8.8 = 800 MPa). The second number times 10 gives yield/tensile ratio percentage (8.8 = 80%, so yield = 640 MPa). Grade 8.8 is standard for automotive, 10.9 for high-strength applications, 12.9 for maximum strength but lower ductility. Proof load is approximately 90% of yield strength. Grade marking is stamped on bolt head for identification.