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:

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.
Why does the same torque give different clamp forces on different bolts?
Because friction varies. The torque-tension formula (T = K × F × d) shows that friction coefficient K directly determines clamp force. Dry steel (K≈0.15), lubricated steel (K≈0.10), and stainless steel (K≈0.25) produce vastly different clamp forces at the same torque. Always use the correct K value for your specific surface condition.
What torque should I use for critical applications?
For critical joints, torque alone is insufficient due to preload scatter. Use angle-controlled tightening (turn-of-nut), hydraulic tensioners, or ultrasonic measurement. If torque is the only option, calibrate the procedure with actual load cells on sample bolts, use lubricated surfaces to reduce friction variability, and apply the torque in multiple passes in a crisscross pattern.
How do I know if a bolt is properly tightened?
You can't determine clamp force from torque alone with certainty. A properly calibrated torque wrench ensures you've applied the correct torque, but actual preload varies ±25–30% due to friction. For verification: check that the bolt hasn't yielded (no permanent elongation), verify the torque was applied smoothly, and for critical joints, use ultrasonic measurement or load-indicating washers.
Can I reuse high-strength bolts?
Grade 8.8 and above bolts are generally designed for single use in structural applications because tightening stretches them near their yield point. Reusing them risks insufficient preload or failure. For non-critical applications, inspect for damage, ensure threads are clean, and never reuse bolts that show signs of yielding, corrosion, or thread damage. Always follow manufacturer guidelines.

How to Use the Bolt Torque Calculator

  1. Select the bolt size — choose from M3 through M36 metric sizes using the dropdown.
  2. Choose the bolt grade — options from 4.8 (low strength) to 12.9 (high strength) per ISO 898-1.
  3. Pick the surface/material — dry steel, zinc-plated, stainless, or aluminium. This auto-sets the friction coefficient (K).
  4. Adjust the friction coefficient manually if needed for lubricated or special conditions.
  5. Read the results — tightening torque (N·m), clamp force (kN), proof load stress (MPa), and tensile stress area (mm²) are displayed instantly.

Understanding Bolt Torque and Preload

When you tighten a bolt, you're converting rotational torque into axial tension — the clamp force or preload that holds the joint together. The fundamental torque-tension relationship is T = K × F × d, where T is torque, K is the friction-dependent nut factor, F is the clamp force (preload), and d is the nominal bolt diameter. The challenge is that torque is an indirect measure of preload: only about 10–15% of the input torque actually creates clamp force, while the rest overcomes thread friction (≈40%) and bearing surface friction (≈50%).

This means friction variability is the dominant source of uncertainty in torque-controlled tightening. A dry, as-rolled steel bolt might have a K factor of 0.15, while the same bolt with oil lubrication drops to 0.10–0.12, and stainless steel can reach 0.25–0.30. This 2–3× variation in friction translates directly to preload scatter of ±25–30% even with precise torque wrenches. For critical applications, engineers use angle-controlled tightening (turn-of-nut method), hydraulic tensioners that stretch the bolt directly, or ultrasonic bolt measurement for the most accurate preload control.

Bolt grades per ISO 898-1 tell you the strength. The grade designation uses two numbers separated by a decimal: the first is 1/100 of the minimum tensile strength in MPa (e.g., 8.8 → 800 MPa), and the second is the ratio of yield to tensile strength (e.g., 8.8 → 0.8 × 800 = 640 MPa yield). Grade 4.8 is common for general-purpose low-strength applications, 8.8 for structural and machinery, and 10.9–12.9 for high-strength critical fasteners. The proof load — the maximum stress with no permanent deformation — is typically 90% of yield, and the design preload target is usually 75% of proof load.

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