The numbers stamped on a bolt head, such as 8.8 or 10.9, are its property class. They tell you how strong the bolt is, so you can match it to the load it will carry.
What the two numbers mean
- The first number, multiplied by 100, is the minimum tensile strength in MPa.
- The second number is the ratio of yield strength to tensile strength, as a tenth.
So an 8.8 bolt has a minimum tensile strength of 800 MPa and a yield strength of 0.8 × 800 = 640 MPa. A 10.9 bolt is 1000 MPa tensile and 900 MPa yield.
Common grades
| Grade | Tensile (MPa) | Yield (MPa) | Typical use |
|---|---|---|---|
| 4.6 | 400 | 240 | Light, non-critical fixing |
| 8.8 | 800 | 640 | General engineering, the usual choice |
| 10.9 | 1000 | 900 | High-load joints in machinery |
| 12.9 | 1200 | 1080 | Socket head cap screws, tooling and dies |
Match the nut to the bolt
Use a nut of equal or higher property class than the bolt. A Class 8 nut suits an 8.8 bolt, and a Class 10 nut suits a 10.9 bolt. A weaker nut can strip before the bolt reaches its rated load.
Stainless steel is about corrosion, not strength
Stainless fasteners are marked A2-70 or A4-80. A2-70 has a minimum tensile strength of 700 MPa, which is lower than 8.8. Choose stainless when corrosion resistance matters more than strength.
Need help choosing?
Send us the application, size and quantity on WhatsApp and we will suggest a suitable grade and finish. See the grade guide for the full table and the size chart for ISO dimensions.