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.
Results
| 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: Rm = 8 × 100 = 800 MPa, Rp0.2 = 8 × 8 × 10 = 640 MPa.
Proof Load Calculation
The proof load is the maximum force a bolt must withstand without permanent deformation:
Where Sp is the proof stress and As is the tensile stress area per ISO 898-1.
Ultimate Tensile Load
Given: Class 8.8, M12 (As = 84.3 mm²), d ≤ 16 mm → Sp = 580 MPa
Proof load Fp = 580 × 84.3 = 48,894 N ≈ 48.9 kN
Tensile load Ft = 800 × 84.3 = 67,440 N ≈ 67.4 kN
⚠️ Note: For class 8.8, Sp = 580 MPa for d ≤ 16 mm and Sp = 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 designation is a coded system per ISO 898-1:
- First number × 100 = nominal tensile strength Rm in MPa
- First × second × 10 = nominal yield strength Rp0.2 in MPa
For class 8.8: Rm = 8 × 100 = 800 MPa, Rp0.2 = 8 × 8 × 10 = 640 MPa.
The second digit thus represents the yield-to-tensile ratio (0.8 = 80%).
Rp0.2 (yield strength) is the stress causing 0.2% permanent elongation — determined by tensile testing.
Sp (proof stress) is a lower, guaranteed minimum stress the bolt must withstand with zero permanent set — used for proof load testing per ISO 898-1.
Typically Sp ≈ 90–95% of Rp0.2. The proof load Fp = Sp × As is the key parameter for bolt specification.
8.8 is the most common class for general engineering:
- Good strength (Rm = 800 MPa) with adequate ductility
- Suitable for most machine frames, flanges, brackets
- Cost-effective, readily available
10.9 is used when higher clamp force is needed:
- Automotive engine/chassis (OEM standard)
- Structural steel connections (EN 14399)
- High-vibration environments requiring higher preload
- Space-constrained joints (smaller bolt, same force)
12.9 is for the most demanding applications where maximum preload per bolt is critical, e.g. precision machinery, motorsport.
Per ISO 898-1, hex bolts must be permanently marked with:
- Property class — e.g. “8.8”, “10.9”, “12.9”
- Manufacturer’s ID — symbol or code
Marking location: top or side of hex head. Socket head cap screws: side of head or in the recess.
Unmarked bolts should be assumed ≤ class 4.6 and must not be used in structural applications.
Proof load Fp = Sp × As
- Sp = proof stress (MPa) from ISO 898-1 for the chosen property class
- As = tensile stress area (mm²) for the thread size per ISO 898-1
- Result is in Newtons (÷ 1000 for kN)
The proof load test per ISO 898-1 applies this force axially for 15 seconds. The bolt must not show permanent elongation.
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.
Professional field balancing instruments and software. Used in 50+ countries.
