How accurate is the K-factor equation?

It is a screening method. Friction variation can create substantial preload scatter even when torque is controlled accurately.

What preload percentage should I use?

There is no universal value. Joint separation, fatigue, gasket sealing, bolt reuse, temperature and proof strength all matter.

Can I use this for structural bolts?

Use the governing structural standard and specified installation method. Structural bolting often uses calibrated wrench, turn-of-nut or tension-control procedures.

Why does lubrication reduce required torque?

Lubrication lowers friction. More of the applied torque becomes bolt tension, so the same torque can over-tighten a lubricated fastener.

Drawing No. EH–ME–004 // Mechanical Engineering

Bolt Torque & Preload Calculator

Reviewed August 2026

Estimate bolt tightening torque from target preload—or preload from applied torque—using metric or inch bolt sizes, grades, proof load and lubrication condition.

Scope: This is a torque–preload screening model, not a substitute for joint design, manufacturer torque specifications or a validated assembly procedure.

What problem does this solve?

Estimate tightening torque from a desired bolt preload, or estimate preload from an applied torque. The tool includes common metric sizes, property classes, lubrication factors and bolt stress areas.

1. Pick boltSelect size and property class.

2. Choose joint conditionDry, plated and lubricated threads use different nut factors.

3. Set preloadUse a target fraction of proof load or enter a force directly.

4. Review uncertaintyTorque is only an indirect estimate of clamp force.

Inputs

Calculation mode

Bolt size

Property class / grade

Bolt diameter mm

Tensile stress area mm²

Proof stress MPa

Target preload % of proof load

Thread / bearing condition

Nut factor K

Direct preload, optional kN

Check the bolt dimensions, proof stress and nut factor.

Results

Proof load

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Target / estimated preload

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Tightening torque

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Bolt tensile stress

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Preload / proof load

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Approx. clamp force

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Important: Torque scatter can be large because most tightening torque is consumed by thread and bearing friction. Critical joints should use a validated tightening procedure, direct tension measurement, torque-angle, tensioning or manufacturer data.

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Background

How the torque–preload relationship works.

Frequently asked questions

Practical questions about inputs, assumptions and interpretation.