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Bolt torque calculator

Choose thread, size, grade and lubrication to get the target preload and tightening torque, a table for every size and how much preload varies with actual friction.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

Pre-sizing
Bolt
Tightening
—Tightening torque
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Torque by size in the same grade

Tightening torque for each size with the same friction and target preload.

Tap the chart to see the values.

Preload obtained versus actual friction

With torque fixed, actual friction changes the preload. The band shows the scatter entered.

Tap the chart to see the values.

Torque table

The report gathers this page's inputs, results, equations, assumptions and references, with fields for project, engineer and revision. In the print dialog, choose Save as PDF.

Method, assumptions and references

Equations used

Metric stress area: As = π/4 · (d − 0.9382·P)²; UNC: As = 0.7854 · (D − 0.9743/n)²
Preload: F = target % × proof stress × As
Nut factor: T = K · F · d
VDI 2230: T = F · (0.16·P + 0.58·d2·µG + Dkm/2·µK), with d2 = d − 0.6495·P and Dkm = (s + dh) ÷ 2
Scatter: with T fixed, F = T ÷ (K·d), for K from (1 − Δ) to (1 + Δ) of nominal

Assumptions and limits

  • Proof stresses from ISO 898-1 (5.6: 280 MPa; 8.8: 580 MPa up to M16 and 600 MPa above; 10.9: 830 MPa; 12.9: 970 MPa) and SAE J429 (Grade 2: 55/33 ksi; Grade 5: 85/74 ksi; Grade 8: 120 ksi); for ASTM A193 B7, the 105 ksi yield strength.
  • In VDI 2230 the bearing surface uses the ISO 4017 hex width across flats and the ISO 273 medium hole; washers and flanges change the friction diameter.
  • Does not check combined torsional stress during tightening, preload loss from embedding or the strength of the internal thread.

Standards and technical documents

  • ISO 898-1:2013: Mechanical properties of fasteners made of carbon steel and alloy steel — Bolts, screws and studs
  • VDI 2230 Blatt 1:2015: Systematic calculation of highly stressed bolted joints
  • ISO 16047:2005: Fasteners — Torque/clamp force testing
  • SAE J429:2014: Mechanical and material requirements for externally threaded fasteners
  • ASME PCC-1-2022: Guidelines for pressure boundary bolted flange joint assembly
  • ISO 261, ISO 4017, ISO 273 e ASME B1.1

U.S. regulations

  • OSHA 29 CFR 1910.119(j): written procedures for maintaining mechanical integrity, including flange bolting in covered processes
  • ASME B31.3 and BPVC Section VIII: bolted joint assembly referenced by state codes

Frequently asked questions

What torque for a 1/2"-13 Grade 5 bolt?

At 75% of proof stress and a nut factor of 0.18 (lightly oiled steel), preload is about 9,000 lbf and torque is around 67 lbf·ft. With anti-seize paste (K ≈ 0.12) it drops to about 45 lbf·ft for the same preload.

Why does friction change preload so much?

Up to 90% of the torque goes into overcoming friction in the thread and under the head; only the rest stretches the bolt. A 20% change in friction shifts preload by about 20%.

Nut factor or VDI 2230?

The nut factor is practical and suits maintenance tables. VDI 2230 separates thread and head friction and is the reference for critical joints and design.