Bolt Tensile Strength Calculator
Calculate maximum tensile load and proof load for a bolt from diameter, thread pitch, and grade.
Supports SAE Grade 5, Grade 8, and metric bolts.
Bolt Tensile Strength
This calculator estimates the maximum static tensile load that a bolt can carry before failure, plus the lower proof load (the safe operating limit). It is based on the bolt’s tensile stress area, which depends on nominal diameter and thread pitch.
Tensile Stress Area (Aₜ)
Aₜ = (π/4) × (d − 0.9382 × p)²
Where d = nominal diameter (mm) and p = thread pitch (mm). This formula is the ISO and ASME standard and accounts for the reduced cross-section at the threads — the weakest point of any bolt under axial load.
Strength Formulas
| Quantity | Formula |
|---|---|
| Ultimate tensile load | Fᵤ = Aₜ × σᵤ |
| Proof load | Fₚ = Aₜ × σₚ |
| Yield load | Fᵧ = Aₜ × σᵧ |
The proof load is the maximum load a bolt can withstand without acquiring permanent set — typically 85–90% of the yield strength.
Common Bolt Grades
| Grade | Tensile σᵤ (MPa) | Proof σₚ (MPa) | Use |
|---|---|---|---|
| SAE Grade 2 | 510 | 380 | Low-strength general |
| SAE Grade 5 | 830 | 580 | Automotive, machinery |
| SAE Grade 8 | 1040 | 830 | High-strength structural |
| Metric 8.8 | 800 | 580 | General engineering |
| Metric 10.9 | 1040 | 830 | Heavy machinery |
| Metric 12.9 | 1220 | 970 | Critical / aerospace |
Worked Example — M10 × 1.5 Grade 10.9
- d = 10 mm, p = 1.5 mm
- Aₜ = (π/4) × (10 − 0.9382 × 1.5)² = (π/4) × 8.59² ≈ 58 mm²
- Ultimate load = 58 × 1040 = 60 300 N ≈ 6.15 tonnes
- Proof load = 58 × 830 = 48 100 N ≈ 4.91 tonnes
Design Considerations
Always design fastened joints to operate well below the proof load. A common rule of thumb is to apply a working preload of 70–80% of proof load and a service-load safety factor of at least 2. Shear loads, bending, fatigue, corrosion, and heat all reduce real-world capacity — these numbers are static, single-bolt limits only.
Limitations
This calculator gives single-bolt static tensile capacity for a fully threaded engagement. It does not account for joint friction, gasket compression, fatigue cycles, hydrogen embrittlement, or torque-tension scatter. For safety-critical applications, consult engineering codes (ASME, ISO 898, AISC) and qualified design data.