This is the indication of the mechanical properties for bolts. In this case it is 8.8, but it can also be 4.6, 4.8, 10.9 or 12.9.

The first number corresponds to 1/100 of the nominal tensile strength (Rm) in N / mm². With an 8.8 bolt, the tensile strength is therefore 8 x 100 = 800 N / mm².

The second number is the ratio of the nominal yield strength (Re) to the nominal tensile strength (Rm) times 10.

Multiplying both numbers yields 1/10 of the nominal yield strength. With our 8.8 bolt, the Re is therefore (8 x 8) x 10 = 640 N / mm².

For the calculation of the bolt surface, you start from the core diameter. So that is different from the outside diameter!

Basically, the higher the number the stronger the bolt.