Bolt grade chart: 8.8 vs 10.9 vs 12.9.

The little numbers stamped on a bolt head — 8.8, 10.9, 12.9 — are the ISO 898-1 property class, and they tell you exactly how strong the fastener is. This chart explains what each number means, the proof and tensile strength of each grade in MPa, how to read the head marking, and how to choose the right class for your joint.

What the numbers actually mean

A metric property class is two numbers separated by a dot, and both are decodable on sight:

• First number × 100 = nominal tensile strength in MPa. So the 8 in 8.8 means about 800 MPa ultimate tensile strength.
• Second number = yield-to-tensile ratio × 10. The 8 after the dot in 8.8 means yield is 80% of tensile, so about 640 MPa.

That is the whole code. A 10.9 bolt is roughly 1000 MPa tensile with yield at 90% (about 900 MPa). A 12.9 bolt is roughly 1200 MPa tensile with yield around 1080 MPa. Once you internalise this, you never need to look up the basic strength again.

The bolt grade chart

Mechanical properties per ISO 898-1 for the common steel property classes:

Values are nominal minimums; exact figures vary slightly with diameter band. The proof load you care about for preload is the proof stress times the tensile stress area of the thread.

Turning strength into clamp force

Grade sets the strength, but the usable preload depends on the thread stress area. For an M10 coarse bolt the tensile stress area is about 58 mm². Multiply by proof stress to get the proof load:

• M10 8.8: 58 × 580 ≈ 33.6 kN proof load
• M10 10.9: 58 × 830 ≈ 48.1 kN proof load
• M10 12.9: 58 × 970 ≈ 56.3 kN proof load

Target preload is usually 65–75% of proof load, and that preload is what your tightening torque has to deliver. To convert grade and size into a torque figure, use the bolt torque calculator, and read the method behind it in our bolt torque guide.

Reading the head marking

Metric structural bolts are marked on the head with the property class itself — you will literally see "8.8", "10.9", or "12.9" stamped on top, often alongside the manufacturer's identifier. Stainless fasteners use a different code such as A2-70 or A4-80, where the number after the dash is one-tenth of the tensile strength in MPa (A2-70 is about 700 MPa). If a bolt head has no marking at all, treat it as unrated and keep it out of any structural joint.

Choosing the right grade

Pick the class from the joint duty, not habit:

• 8.8 — the workhorse. General machinery, structural steel, brackets, guards. Good ductility and forgiving.
• 10.9 — higher clamp load in the same size: engine mounts, drivetrain, heavily loaded flanges.
• 12.9 — maximum strength in tight envelopes: tooling, dies, hydraulic blocks, precision machine joints in clean, static conditions.
• Stainless A2/A4 — when corrosion beats raw strength, at lower mechanical grades.

Heat treatment and plating matter too: zinc-electroplated high-grade bolts can suffer hydrogen embrittlement, so 12.9 fasteners are often specified with alternative coatings. The material density and weight of the fastener stock can be checked with the material weight calculator when you are estimating bulk fastener loads.

Metric class vs SAE grade

These are approximate cross-references only. The standards differ on dimensions, thread tolerance, and test methods, so always work to the grade the drawing specifies rather than substituting on tensile strength alone. For thread sizing and pitch, see the thread pitch reference.

Common fastener-grade mistakes

1. Defaulting to 12.9 for safety. Brittleness and embrittlement risk often make a lower grade the safer engineering choice.
2. Ignoring the nut grade. A class-8 nut on a 12.9 bolt strips before the bolt yields. Match or exceed the nut property class.
3. Using unmarked bolts structurally. No head mark means no rating. Do not assume.
4. Cross-substituting metric and SAE on strength. Equivalents are approximate; honour the drawing callout.
5. Forgetting coating effects. Plating high-grade bolts can introduce hydrogen embrittlement; specify the coating with the grade.