Basis for standard metric thread lengths is..
<= 125mm nominal length; length of thread 2d+6mm
125 - 200mm; length of thread 2d+12mm
200mm+; length of thread 2d+25mm
(d=nominal diameter)
Bolts are specified as x.y, where...
x = 1/10 minimum tensile strength in kgf/mm˛
y = 1/10 ratio of stress at permanent set limit to minimum tensile strength x 100
So an 8.8 bolt has 80 kgf/mm˛ minimum strength, and (1/10)x(64/80)x100 = 8 second digit.
Or a 10.9 bolt has 100/mm˛ minimum strength and (1/10)x(90/100)x100 = 9 second digit.
Multiplying the two numbers gives the stress at the permanent set limit, otherwise known as the yield stress. So, the yield stress on a 10.9 bolt is 90 kgf/mm˛.
The specifying standard is ISO/DR911 for bolts.
For nuts, the strength grade is a single number, and it's a tenth of the proof load stress in kg/mm˛. So a grade 12 nut has a proof load stress is 120 kgf/mm˛. This is in BS3692.
The recommended matching of nuts to bolt is...
Bolt Nut
4.6 4
4.8 4
5.6 5
5.8 5
6.6 6
6.8 6
8.8 8
10.9 12
12.9 12
14.9 14
There's also a range of Vickers hardenesses permitted for the ranges of bolts, specifically: -
4.8 110 - 170
5.8 140 - 215
6.8 170 - 245
8.8 225 - 300
10.9 250 - 370
12.9 330 - 440
14.9 400 - 510
Hope this helps, but none of it changes what I said before, take care!
G