If you have ever looked at the head of a bolt and seen a small marking like “8.8” or “10.9”, you were looking at its property class — the fastener’s strength passport. These two numbers tell you how strong the bolt is, what it is made of, and whether it is suitable for a high-load structural joint or a light assembly.
Choosing the wrong grade is one of the most common causes of fastener failure in service. A bolt that is too weak can shear under load; one that is needlessly strong costs more than necessary and may be brittle. This guide explains what 8.8, 10.9 and 12.9 really mean, how they compare, and how to specify the right grade for your application.What Do the Numbers on a Bolt Mean?
For metric bolts made to ISO 898-1, the two numbers are not random:
- The first number is 1/100 of the minimum tensile strength (Rm) in MPa. A grade 8.8 bolt has a minimum tensile strength of 800 MPa.
- The second number is the ratio of yield strength to tensile strength, multiplied by 10. For 8.8, the ratio is 0.8 — meaning the yield strength is 640 MPa (80% of 800).
So “10.9” tells you: 1,000+ MPa tensile strength and a yield/tensile ratio of 0.9. Because these numbers follow this logic, you can read a bolt’s strength directly from its head marking.Bolt Grade Chart (ISO 898-1)
The table below summarizes the mechanical properties of the most commonly specified metric property classes for bolts, screws and studs (applies to coarse-pitch threads, sizes M5–M39, per ISO 898-1):
| Property class | Min. tensile strength Rm (MPa) | Min. yield strength (MPa) | Yield/tensile ratio | Typical material & treatment | Typical head marking |
|---|---|---|---|---|---|
| 4.8 | 400 | 340 | 0.80 | Low or medium carbon steel | 4.8 |
| 5.8 | 500 | 420 | 0.80 | Low or medium carbon steel | 5.8 |
| 6.8 | 600 | 480 | 0.80 | Medium carbon steel | 6.8 |
| 8.8 | 800 | 640 | 0.80 | Medium carbon or alloy steel, quenched & tempered | 8.8 |
| 10.9 | 1,040 | 940 | 0.90 | Alloy steel (Cr, Cr-Mo), quenched & tempered | 10.9 |
| 12.9 | 1,220 | 1,100 | 0.90 | Alloy steel (Cr-Mo, Cr-Ni-Mo), quenched & tempered | 12.9 |
Values are minimums per ISO 898-1:2013. Yield strength for classes 8.8 and above is measured as 0.2% proof stress (Rp0.2).Material and Heat Treatment Behind Each Grade
The property class is not only a strength value — it implies a material and a manufacturing route:
- 4.8 / 5.8: low or medium carbon steel, as-produced. These are general-purpose bolts for light-duty and non-critical applications.
- 6.8: medium carbon steel, suitable for moderately loaded connections.
- 8.8: medium carbon steel or alloy steel, quenched and tempered. This is the most widely used high-strength grade, balancing strength, toughness and cost.
- 10.9: alloy steel (chromium or chromium-molybdenum), quenched and tempered. Used where higher preload and fatigue resistance are required.
- 12.9: higher-alloy steel, quenched and tempered to maximum strength. Highest strength of the standard classes — but also the least ductile, so careful attention to embrittlement (especially under plating processes) is essential.
A rule worth remembering: the stronger the grade, the lower the ductility. 12.9 bolts are extremely strong but tolerate less misalignment, shock and hydrogen embrittlement risk than 8.8.How to Identify a Bolt Grade: Head Markings
Every bolt that meets ISO 898-1 carries a marking on its head:
- 8.8 bolts are marked “8.8”.
- 10.9 bolts are marked “10.9”.
- 12.9 bolts are marked “12.9”.
For larger sizes, the marking is usually stamped on the head; smaller bolts may use a symbol or the marking may be omitted where space does not allow — in which case you should rely on the supplier’s documentation and test reports. If a bolt claimed to be 10.9 shows no marking and the supplier cannot provide material certificates, treat it as a red flag.
Inspection tip: for critical applications, verify the grade with hardness testing (HRC) and, where required, a tensile test rather than trusting the marking alone.What About Stainless Steel Bolts?
Stainless steel bolts follow a different classification system under ISO 3506, which uses an “A” or “C” group plus a strength class:
| Class | Typical steel | Min. tensile strength (MPa) | Typical use |
|---|---|---|---|
| A2-70 | Stainless 304 (austenitic, cold worked) | 700 | General corrosion-resistant applications |
| A4-70 | Stainless 316 (austenitic, cold worked) | 700 | Marine, chemical and chloride environments |
| A4-80 | Stainless 316 (heavily cold worked) | 800 | Higher-strength stainless applications |
For stainless bolts, the letter (A2 or A4) identifies the material family and the number (70, 80) the tensile strength class. A2-70 is the most common stainless bolt grade; A4 grades add molybdenum for superior resistance to chlorides and are the standard choice for marine and chemical environments.How to Choose the Right Grade
Match the grade to the demands of the joint:
1. What loads will the joint carry? Static structural loads, dynamic/fatigue loads and preload requirements all raise the strength needed. For general steel construction and machinery, 8.8 is the default high-strength choice.
2. What does the rest of the system use? Mating components (nuts, washers) and the base material should be compatible with the bolt grade — a 12.9 bolt into a soft base material adds little value.
3. What is the environment? For corrosion exposure, select stainless (A2/A4) or a coated carbon steel rather than simply raising the strength class.
4. Is ductility important? Vibration, shock or misalignment favor 8.8 over 12.9 despite the lower strength, because ductility and toughness matter in those conditions.
Guidance: start with 8.8 for most machinery and structural work, move to 10.9 when higher preload or fatigue resistance is required, and reserve 12.9 for specialized high-strength designs where its limited ductility is understood and controlled.Common Mistakes When Specifying Grades
Hydrogen embrittlement: high-strength classes (especially 10.9 and 12.9) are more sensitive to plating processes. Specify low-embrittlement coating processes for these grades.
Confusing strength with corrosion resistance: a high-grade carbon steel bolt is strong but will rust. For corrosive environments, choose stainless or a proper coating.
Overspecifying: using 12.9 where 8.8 suffices raises cost and embrittlement risk without benefit.
Ignoring the nut: the nut’s property class must match the bolt (a softer nut limits the whole assembly).
Skipping documentation: for critical bolts, require material certificates and test reports — the marking alone is not enough.Frequently Asked Questions
Is a higher grade always better?
No. Higher grades are stronger but less ductile and more sensitive to manufacturing processes. Choose the grade that matches the application’s load and environment — 8.8 covers most industrial uses.
What does the “8” and the “8” in 8.8 mean separately?
The first “8” = 800 MPa minimum tensile strength; the second “8” = 0.8 yield/tensile ratio (yield strength 640 MPa).
Can I use a 10.9 bolt with an 8.8 nut?
Nut classes should match or exceed the bolt class (nut 10 class for an 8.8 bolt is common). A weaker nut will limit the assembly’s capacity.
How do I verify the grade of a delivered bolt?
Check the head marking, request the material certificate and batch test report, and for critical parts perform hardness or tensile verification.Need the right grade for your project?
Specifying fasteners correctly is half the battle — finding a certified supplier who delivers exactly that specification is the other half. FASTENERHALL connects you with certified fastener manufacturers directly connected to source factories, covering bolts, nuts, screws, washers, retaining rings, pins and custom parts.
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