Bolt strength is determined by either class or grade. For classes, the number before the point is the tensile ultimate strength (the strength at which the bolt fails) divided by 100. The number after the point is 10 times the ratio of yield strength (the load at which the bolt will be permanently deformed) to ultimate tensile strength. For example, Class 8.8 has a minimum tensile strength of 800 MPa and a yield strength of 0.8 times 800, or 640 MPa.
Grade indicates strength by declaring the quality of the material which the bolt is made of. Grades range from 0 to 8, with 8 being the strongest. Lower-grade bolts have a higher chance of failure, as they are made from lower-quality material. Grades 5 and 8 are the most common.