Bolt Torque Calculator — Tightening Torque & Bolt Preload
Enter the bolt nominal diameter, the target clamping (preload) force, and the nut factor for your lubrication condition to get the required tightening torque in N·m, lbf·ft, lbf·in and kgf·cm.
mm
kN
Nut factor K (friction condition)
Torque to achieve the target clamping force via T = K × F × d
- 1
Diameter in metres
12 ÷ 1000 = 0.012 - 2
Clamping force in Newtons
20 × 1000 = 20,000 - 3
Tightening torque
0.2 × 20,000 × 0.012 = 48T = K × F × d, where K bundles thread and bearing-face friction.
How does this calculator work?
Tightening torque is T = K × F × d, where K is the nut factor (0.10–0.30 depending on lubrication), F is the clamping force in Newtons, and d is the bolt nominal diameter in metres. Lubrication dramatically reduces K — mismatching K to your actual surface condition can cause significant over- or under-preloading.
Formula
How this is calculated
When a bolt is tightened, the applied torque T does two things: it overcomes friction in the threads and under the nut (or bolt head) bearing face, and it stretches the bolt shank to create the clamping (preload) force F. The standard torque-tension equation captures this as T = K × F × d, where d is the nominal bolt diameter and K is a dimensionless nut factor (also called the torque coefficient) that bundles thread geometry, thread friction and bearing-face friction into one number.
The nut factor K is not a fixed material constant — it depends on thread finish, lubrication, and surface coating. Typical values range from about 0.10 (waxed or heavily lubricated) to 0.30 (rough or corroded threads), with 0.20 being the widely used default for clean, unlubricated steel on steel. Applying a thread lubricant or anti-seize compound significantly reduces K and therefore the torque needed; if an engineer's torque specification assumed a dry condition (K ≈ 0.20) and you apply anti-seize (K ≈ 0.13), you can over-preload the joint by up to 50%.
This calculator gives a first-principles estimate. Real joint designs must account for bolt grade proof strength, joint stiffness, embedment relaxation and re-tightening effects — always verify against the manufacturer's or design engineer's torque specification before use in critical assemblies.
Frequently asked questions
The nut factor K lumps thread geometry, thread friction and bearing-face friction into a single dimensionless coefficient. It is not a fundamental material property — it changes with thread type (coarse vs fine), lubrication, surface finish, plating (zinc, cadmium, PTFE) and even temperature. Values from 0.10 (waxed) to 0.30 (corroded) cover most practical situations. Always use the K value specified in the joint design or the fastener manufacturer's data sheet, especially for structural applications.
Service manual torque specs are derived from testing for the specific assembly — fastener grade, thread lubricant, joint material and target preload. This calculator gives a theoretical estimate from the torque-tension equation that engineers use in preliminary design. For critical assemblies — engine cylinder heads, wheel studs, brake callipers — always use the manufacturer's tested and validated torque value instead of relying on this estimate alone.
Roughly 40–50% of tightening torque fights thread friction, and another 30–40% fights bearing-face friction; only about 10–15% actually stretches the bolt and creates clamping force. Lubrication lowers these friction components (lower K), so less torque is needed for the same preload. Ignoring lubrication and applying a dry-condition torque value to a lubricated joint can generate dangerously high preloads or even break the fastener.
TG we-Calculate Editorial Team. (2026). Bolt Torque Calculator — Tightening Torque & Bolt Preload [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/bolt-torque-calculator
TG we-Calculate Editorial Team. "Bolt Torque Calculator — Tightening Torque & Bolt Preload." TG we-Calculate. 2026. https://we-calculate.com/calculator/bolt-torque-calculator.
TG we-Calculate Editorial Team, "Bolt Torque Calculator — Tightening Torque & Bolt Preload," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/bolt-torque-calculator
@misc{wecalculate_bolt_torque_calculator, title = {Bolt Torque Calculator — Tightening Torque & Bolt Preload}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/bolt-torque-calculator}}, year = {2026}, note = {TG we-Calculate} }
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