Engineering Reference

Bolt Torque Calculator

Compute the tightening torque needed to reach a target preload, and the preload produced by a given torque.

Page last changed 2026-10-19 · based on n/a — standard engineering relationships, no single governing revision

Quick Answer

Torque and preload are related by T = K × D × F, where K is the nut factor — about 0.20 for plain steel fasteners, lower when lubricated. Lubrication changes the preload for the same torque by a factor of two, which is why torque specs always state the condition.

Bolt Torque and Preload

The Formulas Used

T = K × D × F
T is torque, K the nut factor (≈0.20 plain, ≈0.16 lubricated, ≈0.12 waxed), D the nominal diameter and F the target preload. Preload is normally set at 75% of the bolt proof load.
As = (π/4) × (D − 0.9743/n)² for the tensile stress area.

The Nut Factor Is the Uncertainty

The formula is simple; K is not. It folds together thread friction, under-head friction and the geometry of the thread, and it varies from about 0.12 for a waxed fastener to 0.20 or more for a dry, plated one. Because torque is proportional to K, the same torque can produce nearly twice the preload on a lubricated bolt as on a dry one.

This is why torque specifications always state the condition, and why critical joints are tightened by torque-plus-angle or by measuring bolt stretch rather than by torque alone. Torque is a proxy for preload, and a loose one — scatter of ±25% is normal.

Why 75% of Proof Load

A bolt is normally preloaded to about 75% of its proof load. Higher risks yielding the fastener during tightening or under service load; lower leaves the joint vulnerable to fatigue and to loosening, because a joint that separates under load puts the full cyclic load on the bolt.

Frequently Asked Questions

How do I calculate bolt torque?
T = K × D × F, where K is the nut factor (about 0.20 for plain steel), D is the nominal diameter and F is the target preload — normally 75% of the bolt's proof load.
What is the nut factor K?
A dimensionless factor combining thread friction, under-head friction and thread geometry. It runs from about 0.12 for a waxed fastener to 0.20 or higher for a dry plated one — which is why lubrication changes preload so much at the same torque.
Why is a lubricated bolt easier to over-tighten?
Lubrication lowers K, so the same torque produces more preload. A bolt tightened to a dry-torque figure but lubricated can be loaded well past its yield point and fail.
What is the difference between torque and preload?
Torque is what you apply with a wrench; preload is the tension that torque produces in the bolt. Only about 10–15% of the applied torque goes into stretching the bolt — the rest is lost to friction.
How accurate is torque as a measure of preload?
Not very — scatter of ±25% is normal, because the nut factor varies with lubrication, plating, surface finish and reuse. Critical joints use torque-plus-angle or direct bolt-stretch measurement instead.

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