About this bolt torque calculator
Bolt torque calculator
Bolt Torque Calculator and Bolt Pretension Calculator
Calculate metric and unified bolt torque, bolt pretension, API-style tightening torque, friction effects, retained clamp, and report-ready bolting outcomes.
Use this bolt torque calculator to estimate tightening torque from bolt preload, thread geometry, friction, bearing diameter, tightening scatter, preload loss, and service loading. The page is built for engineers who need more than a lookup table: it shows the assumptions behind the torque value, the retained clamp force, and the checks that should be reviewed before a bolting procedure is released.
Common engineering uses
Common engineering uses
- Metric and unified thread torque estimates for machine, structural, and pressure-containing joints.
- Bolt pretension, preload percentage, retained clamp, slip resistance, and simple pressure-opening checks.
- Component clamp-limit checks where a washer, cover, flange face, or soft clamped part governs the safe preload.
Outputs to review
Outputs to review
- Target preload, tightening torque, thread torque share, bearing torque share, and equivalent nut factor.
- Bolt stress, proof and yield utilization, retained clamp after scatter and relaxation, and service load warnings.
- Report-ready calculation notes that make friction, bearing geometry, preload basis, and limitations visible.
How to use this tool
How to use this tool
- Select metric or imperial unified thread size.
- Select bolt material, preload basis, and target preload percent or direct preload.
- Choose friction, bearing geometry, tightening scatter, and preload loss.
- Review torque, bolt stress, remaining clamp, slip resistance, warnings, and PDF report output.
How to use the result
Start with the joint purpose, then choose the thread system, bolt material, preload basis, friction condition, bearing model, tightening scatter, and any service opening or shear load. Treat the torque as an engineering estimate, not a universal shop value. For critical bolting, tie the result to controlled lubrication, calibrated tools, tightening sequence, inspection acceptance, and project-specific review.
Assumptions, limitations, and formulas
Assumptions
Threads are treated as single-start 60-degree metric ISO or unified thread forms.
Torque is calculated from thread helix torque plus nut/head bearing torque.
Pressure-containing checks focus on target preload, face contact risk, retained clamp, and validated make-up procedure review.
Structural checks include a simple slip-resistance comparison from retained clamp, slip coefficient, and number of interfaces.
Limitations
Does not replace official code design, gasket seating qualification, flange stress analysis, fatigue analysis, prying checks, or project bolting procedures.
Torque-preload accuracy depends strongly on actual friction, lubrication, coating, surface finish, and tool calibration.
Critical pressure-boundary, lifting, and safety-related bolting requires qualified engineering review and validated procedures.
Formula notes
Stress area uses standard metric ISO and unified-thread tensile stress area approximations.
Torque = thread torque + bearing torque, using selected thread and bearing friction coefficients.
Service load check uses Fb = Fi + C x P and remaining clamp = Fi - (1 - C) x P after preload loss.
Common questions
How is bolt tightening torque calculated?
The calculator starts from target bolt preload, then estimates thread torque and bearing torque from thread geometry, friction, and bearing diameter. It also reports an equivalent nut factor, so the result can be compared with the familiar T = K x F x d relationship.
Why can the same torque give different bolt preload?
Most tightening torque is spent overcoming friction in the threads and under the nut or head. Small changes in lubrication, coating, surface finish, washer condition, and tool method can change the achieved preload even when the displayed torque is the same.
Can this calculator be used for API flange bolting?
It can support early review of API-style pressure-containing bolting by exposing preload basis, friction, bearing geometry, pressure opening load, and retained clamp. Final API flange bolting work still needs the project standard, approved procedure, qualified tooling, and competent engineering review.
Is the calculated torque a final bolting procedure?
No. The result is a transparent engineering estimate. Critical joints need validated friction values, lubrication control, tightening sequence, calibration, inspection criteria, fatigue or gasket checks where relevant, and approval under the project procedure.