Metric strength grades correspond to the inch strength grades in fasteners

Some details on conversion guidance between metric and inch based strength grades is given in section 3.4 of the standard SAE J1199 (Mechanical and Material Requirements for Metric Externally Threaded Steel Fasteners).

Metric fastener strength is denoted by a property class which is equivalent to a strength grade. Briefly:

• Class 4.6 is approximately equivalent to SAE J429 Grade 1 and ASTM A307 Grade A
• Class 5.8 is approximately equivalent to SAE J429 Grade 2
• Class 8.8 is approximately equivalent to SAE J429 Grade 5 and ASTM A449
• Class 9.8 is approximately 9% stronger than equivalent to SAE J429 Grade 5 and ASTM A449
• Class 10.9 is approximately equivalent to SAE J429 Grade 8 and ASTM A354 Grade BD

For information there is no direct inch equivalent to the metric 12.9 property class.