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.

Bolt Torque Calculator

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

Data verified 2026-09-29 · 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

Nominal diameter (in)

Threads per inch

Tensile strength (psi)

Nut factor K

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.

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

#	Source	Type	Revision / method
[1]	ASME B1.1 — Unified Inch Screw Threads	standard	ASME B1.1-2019 — source
[2]	ASTM A615 — Deformed steel bars for concrete reinforcement	standard	ASTM A615/A615M-20 — source
[3]	ASTM E140 — Hardness Conversion Tables	standard	ASTM E140-12b — source
[4]	Values computed in your browser	derived	Evaluated locally from the formulas shown on the page. No data leaves the device.
[5]	ISO 4287 — Surface texture: Profile method	standard	ISO 4287:1997 — source
[6]	ISO 68-1 — Basic profile	standard	ISO 68-1:2023 — source
[7]	NFPA 70 NEC Table 310.16	standard	NEC 2023 (NFPA 70-2023) — source

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

Cross-checked against:

Formulas verified against the corresponding reference chart on this site, so the calculator and the published tables agree

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.

Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:

Plain text

Engineering Reference. "Bolt Torque Calculator." https://refmanual.com/calculators/bolt-torque/ (accessed 2026-09-29). Data from Formulas as shown on this page, n/a — standard engineering relationships, no single governing revision.

BibTeX

@misc{bolt-torque,
  title        = {Bolt Torque Calculator},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/calculators/bolt-torque/}},
  note         = {Accessed: 2026-09-29},
  source       = {Formulas as shown on this page, n/a — standard engineering relationships, no single governing revision}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - Bolt Torque Calculator
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/calculators/bolt-torque/
PY  - 2026
DA  - 2026-09-29
N1  - Source: Formulas as shown on this page, n/a — standard engineering relationships, no single governing revision
ER  -

Each row in the tables above also has a permanent link — hover a row and use the # link to cite a single value rather than the whole page.