Bolt Torque Calculator
Calculate tightening torque, clamp force, and proof load for metric bolts M3–M36 using ISO 898 stress areas and standard bolt grades.
⚠ Disclaimer: These values are approximate and for reference only. Actual torque values depend on installation conditions, lubrication, hole clearance, and joint design. Always consult the relevant engineering standards and manufacturer specifications for critical applications.
Quick Reference: Common Bolt Torques
Grade 8.8 bolts, steel zinc-plated (K = 0.20). Torque in N·m, clamp force in kN.
| Size | Stress Area (mm²) | Proof Stress (MPa) | Torque (N·m) | Clamp Force (kN) |
|---|
Formula
The torque is calculated using the standard bolt torque formula:
T = K × σproof × As × d / 1000
Where:
- T = tightening torque (N·m)
- K = nut friction coefficient (dimensionless)
- σproof = proof load stress for the bolt grade (MPa)
- As = tensile stress area per ISO 898-1 (mm²)
- d = nominal bolt diameter (mm)
Proof load stress for each grade: 4.8→310 MPa, 5.8→380 MPa, 6.8→440 MPa, 8.8→580 MPa, 9.8→600 MPa, 10.9→830 MPa, 12.9→970 MPa.
FAQ
What friction coefficient should I use?
It depends on the surface finish and lubrication. Dry steel is typically 0.15, zinc-plated steel 0.20, stainless steel 0.25, and aluminium 0.30. Lubricated surfaces can be as low as 0.10–0.12. When in doubt, use the value for your specific surface condition or consult the fastener manufacturer.
What is proof load stress?
Proof load stress is the maximum stress a bolt can withstand without permanent deformation. For a given grade (e.g., 8.8), the proof load stress is 580 MPa. It is derived from the first number × 10 × the second number / 10, then rounded to the standard value from ISO 898-1.
What is tensile stress area (As)?
The tensile stress area is the effective cross-sectional area of the bolt thread, calculated per ISO 898-1. It is slightly less than the nominal shank area because the threads reduce the load-bearing cross-section. For example, an M10 bolt has a nominal area of 78.5 mm² but a stress area of only 58.0 mm².
Should I use torque or tension for critical joints?
Torque is an indirect method of achieving clamp force — friction variability means the actual preload can vary by ±25–30%. For critical joints, direct tensioning (hydraulic tensioners, ultrasonic measurement) or angle-controlled tightening provides more consistent results.
Why is the formula divided by 1000?
The division by 1000 converts the result from N·mm to N·m. Since K is dimensionless, σproof is in MPa (N/mm²), As is in mm², and d is in mm, the raw result is in N·mm. Dividing by 1000 gives the standard torque unit of N·m.