Sources and scope for this page: Formulas as shown on this page (standard engineering relationships, no single governing revision), covering every value this calculator produces. No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. Full sources, values and limits on this page.
Page last changed 2026-11-08 · based on n/a — standard engineering relationships, no single governing revision
Quick Answer
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.
Related
Value Sources
References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.
| # | Source | Type | Revision / method |
|---|---|---|---|
| [1] | Values computed in your browser | derived | Evaluated locally from the formulas shown on the page. No data leaves the device. |
Data Sources
| Standard | Revision | What it covers on this page |
|---|---|---|
| Formulas as shown on this page | n/a — standard engineering relationships, no single governing revision | every value this calculator produces |
Verification scope
No page-specific independent source review is recorded here. The listed references and formulas explain the basis of the compilation; automated consistency checks do not verify the original sources or establish product conformity.
Derived values — the following values on this page are calculated, not taken directly from the standard:
| Value | How it is derived |
|---|---|
| All outputs | Computed in the browser from the formulas above. No data leaves the device. |
Outputs are computed from the formulas shown. Verify against the governing standard for design or acceptance work.
Cite This Page
Read the source notes, calculation methods and verification scope above before reusing a value. To cite this page: