Bolt Torque Calculator
Pick a metric bolt size, property class, and thread condition to estimate the tightening torque and the clamp force it produces — in newton-metres, pound-feet, and kilonewtons.
Recommended tightening torque
How the calculation works
The tool first finds the thread’s tensile stress area from the ISO 898 formula — the effective cross-section that carries load, a little smaller than the nominal diameter. It multiplies that area by the property class’s proof strength and the 75% preload factor to get the target clamp force, then applies the short-form torque relationship T = K·F·d. The nut factor K comes from the lubrication condition you select.
Because torque scales directly with the nut factor and the bolt diameter, a larger or drier bolt needs markedly more torque for the same clamp force. Every figure is rounded for display; the underlying values are carried at full precision.
Frequently Asked Questions
How is bolt torque related to clamp force?
Tightening a bolt stretches it like a stiff spring; that stretch produces the clamp (preload) force that actually holds the joint together. The torque you apply on the wrench does two jobs at once — it creates that preload and it overcomes friction under the bolt head and in the threads. The short-form model T = K·F·d links them, where T is torque, F is preload, d is the nominal diameter, and K is the nut factor that bundles all the friction into one number.
What is the nut factor K and why does lubrication change it?
K is a dimensionless friction coefficient for the whole fastener. A dry, as-received bolt is around 0.20; zinc plating runs a little higher; oil, grease, or moly paste drop it toward 0.12. Because torque is proportional to K, a well-lubricated bolt reaches the same clamp force at a noticeably lower wrench setting — which is exactly why a torque spec is only valid for the friction condition it was written for.
Why does the tool target 75% of proof load?
A common preload target for a reusable structural joint is roughly 75% of the bolt's proof load — high enough to keep the joint clamped and resist loosening, with a margin below the point where the bolt yields. This calculator uses that 75% figure by default; safety-critical or angle-controlled joints may specify a different fraction, so always defer to the design documentation.
Are these torque values a substitute for a manufacturer's spec?
No. The result is a physics-based estimate for a standard ISO metric bolt in a typical joint. Real specifications account for gasket crush, joint stiffness, thread engagement, reuse, and the exact friction condition. Treat this as a sanity check and a learning aid, not as the final authority for a fastener that matters.
Educational tool only. Results are engineering estimates rounded for display; verify any torque that matters for assembly, safety, or certification against the fastener manufacturer’s specification.