Bolt Property Classes Explained: 8.8, 10.9 and 12.9

Updated: 2026-10-28

Metric bolt property classes such as 8.8 and 10.9 encode the bolt's tensile strength and yield strength in two numbers. Steel bolts are covered by ISO 898-1 and are marked with their class on the head.

How to read the numbers

Example: 10.9 - First number 10 × 100 = nominal tensile strength 1,000 MPa - Second number 9 × 10 = yield strength is 90% of tensile - Nominal yield strength = 1,000 × 0.9 = 900 MPa

Common property classes

Class	Nominal tensile	Nominal yield	Typical material and use
4.8	400 MPa	320 MPa	Low-carbon steel, general use
8.8	800 MPa	640 MPa	Medium-carbon or alloy steel, quenched and tempered; machinery, automotive
10.9	1,000 MPa	900 MPa	Alloy steel, quenched and tempered; high-strength joints
12.9	1,200 MPa	1,080 MPa	Alloy steel; socket head cap screws and high-strength uses

These are nominal values. Minimum values in the standard vary with diameter, so check ISO 898-1 for design.

Stainless steel classes

Stainless bolts follow ISO 3506-1, which combines the steel group and strength:

Marking	Meaning
A2-70	Austenitic A2 (304 type), tensile strength ≥ 700 MPa
A4-70	Austenitic A4 (316 type), tensile strength ≥ 700 MPa
A4-80	A4 (316 type), tensile strength ≥ 800 MPa

Selection notes

Higher is not automatically better. Tightening torque, internal thread strength and bearing surface all matter.
10.9 and above with electroplating risk hydrogen embrittlement; consider baking or zinc flake coatings.
For custom bolts with a property class, agree material, heat treatment and test methods according to the standard.

FAQ

What is the difference between 8.8 and 10.9 bolts?

Nominal tensile strength is 800 MPa for 8.8 and 1,000 MPa for 10.9. 10.9 allows higher clamp load, but the mating thread and coating must suit it.

Are there 10.9 stainless bolts?

Stainless bolts use a different system (ISO 3506), marked A2-70, A4-80 and so on. Classes like 10.9 apply to carbon and alloy steel bolts.