What do the two numbers in a bolt class like 8.8 mean?

The first number × 100 gives the nominal tensile strength Rm in MPa. The second number × first number × 10 gives the nominal yield strength Rp0.2 in MPa. For class 8.8: Rm = 8 × 100 = 800 MPa and Rp0.2 = 8 × 8 × 10 = 640 MPa.

What is the difference between Rp0.2 and Sp (proof stress)?

Rp0.2 is the 0.2% offset yield strength — the stress at which the bolt takes a 0.2% permanent set. Sp (proof stress) is a guaranteed minimum stress the bolt must withstand without permanent deformation. Sp is typically 90–95% of Rp0.2 and is used to calculate the proof load for quality testing.

When should I use 10.9 versus 8.8 bolts?

Use 8.8 for general-purpose applications — it offers good strength with ductility. Use 10.9 when higher clamping force is needed in a limited space, such as automotive engine assemblies, structural steel connections, or high-vibration environments. 12.9 is reserved for the most demanding applications where maximum preload per bolt is essential.

How are bolt grades marked on the bolt head?

Per ISO 898-1, hexagon bolts and screws must be permanently marked on the head (top or side) with the property class designation (e.g., 8.8, 10.9, 12.9) and the manufacturer's identification mark. Socket head cap screws may be marked on the side of the head or on the recess.

How is proof load calculated?

Proof load Fp = Sp × As, where Sp is the proof stress (MPa) for the property class and As is the tensile stress area (mm²) for the thread size. The result is in Newtons (N). Divide by 1000 for kN. For example, an M12 class 8.8 bolt: Fp = 580 × 84.3 = 48,894 N ≈ 48.9 kN.

Free Engineering Tool

Bolt Strength Classes (4.6–12.9)

Look up mechanical properties for any bolt property class per ISO 898-1 — tensile strength, yield strength, proof stress, hardness — and calculate proof load for any metric thread size.

This calculator was developed and verified by the Vibromera engineering team, makers of the Balanset portable balancing instruments.

ISO 898-1 Classes 4.6 – 12.9 M3 – M48

Results

Proof Load Fp

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Tensile Strength Rm

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Yield Strength Rp0.2

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Proof Stress Sp

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Stress Area As

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Elongation at Break

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Hardness HRC

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Ultimate Tensile Load

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Class	Rm (MPa)	Rp0.2 (MPa)	Sp (MPa)	Elong. %	HRC	Material

Reading the Property Class Designation

The two-number system (e.g., 8.8) encodes the bolt’s key mechanical properties:

For class 8.8: R_m = 8 × 100 = 800 MPa, R_p0.2 = 8 × 8 × 10 = 640 MPa.

Proof Load Calculation

The proof load is the maximum force a bolt must withstand without permanent deformation:

Where S_p is the proof stress and A_s is the tensile stress area per ISO 898-1.

Ultimate Tensile Load

Example — M12 Class 8.8 Bolt

Given: Class 8.8, M12 (A_s = 84.3 mm²), d ≤ 16 mm → S_p = 580 MPa

Proof load F_p = 580 × 84.3 = 48,894 N ≈ 48.9 kN

Tensile load F_t = 800 × 84.3 = 67,440 N ≈ 67.4 kN

⚠️ Note: For class 8.8, S_p = 580 MPa for d ≤ 16 mm and S_p = 600 MPa for d > 16 mm. This is because larger bolts have a higher ratio of hardenable to total cross-section.

ℹ️ Design tip: Typical bolt preload target is 75–90% of proof load. Never exceed proof load during tightening. Use calibrated torque wrenches or angle-controlled tightening for critical joints.

The procedures described on this page were developed and field-tested by the Vibromera team using the Balanset-1A, a portable dual-channel balancer and vibration analyzer for on-site rotor balancing.