Mechanics · Bolts, preload and tightening

Bolt preload and clamp force calculator

Size the initial preload per bolt as an explicit fraction of its tensile proof load.

Created Aug 13, 2026v1.0

By Thibaut Grzelak, Mechanical Analysis Engineer

Bolts, preload and tightening

Results

Metric

Target preload / theoretical initial clamp force

35.39187kN

Proof load

50.55982kN

Tensile stress area

84.26637mm²

Preload as percent of proof load

70%

View calculation detailsFormulas, SI conversion, numerical substitution and resultsOpen ↓

Calculation note

Full calculation details

Sp is an applicable proof stress supplied by the user. Metric preset area is calculated from nominal diameter and pitch. In the ideal initial model, bolt preload equals the clamp force on the parts. Relaxation, embedment, joint stiffness and external load are excluded.

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01Formulas and symbols

Formulas used

Atπ/4 × (d − 0.9382P)²Metric tensile stress area approximation

FproofSp × AtProof load

Fir × FproofTarget preload per bolt

02Assumptions and limits

Scope of validity

Sp is an applicable proof stress supplied by the user.
Metric preset area is calculated from nominal diameter and pitch.
In the ideal initial model, bolt preload equals the clamp force on the parts.
Relaxation, embedment, joint stiffness and external load are excluded.

03Validation example

Reference numerical case

M12×1.75 gives At ≈ 84.27 mm².
With Sp = 600 MPa, Fproof ≈ 50.56 kN.
At r = 0.70, Fi ≈ 35.39 kN.

04Frequently asked questions

Questions about the calculation

Are preload and clamp force separate results?

Not in this ideal initial model: they are equal and shown as one per-bolt result.

Can Sp be replaced by 90% of ultimate strength?

No. Use a confirmed proof stress for the applicable class, size and scope.

How is an unlisted thread handled?

Select custom mode and enter tensile stress area from an applicable reference.

05References

Technical references

ISO 724:2023, ISO general purpose metric screw threads — Basic dimensions.
ISO 898-1:2013, mechanical properties of steel bolts, screws and studs, confirmed in 2025.
NASA RP-1228, Fastener Design Manual.