That "8.8" or "10.9" on a bolt head is not a size. It's a strength grade, called the property class.

First number × 100 = tensile strength in MPa. Second number × 10 = yield as a percentage of tensile. So 8.8 is 800 MPa tensile and 640 MPa yield.

Yield is the point of no return: below it, a bolt springs back; past it, it stays stretched.

Values shown are nominal. See ISO 898-1 for the exact minimums, and match your nut class to your bolt class.

Next time you hold a bolt, read its head.