Formula used
Metric tensile area: At = π/4 × (d − 0.9382p)².
Imperial tensile area: At = π/4 × (d − 0.9743/n)².
Tensile stress: σt = F/At.
Proof capacity: Fp = AtSp.
Preload: Fi = preload fraction × Fp.
Torque estimate: T = K Fid.
Joint constant: C = kb/(kb + km).
Goodman check: U = σa/Se + σm/Su.
How to use this calculator
Select the calculation mode and preferred output units. Choose metric or imperial thread geometry. Enter the bolt diameter and thread information carefully.
Select a grade preset or enter custom strengths. Add loading, preload, and distribution assumptions. Use realistic factors from approved engineering documentation.
Review every capacity, safety factor, and warning. Compare results with governing design requirements. Confirm final selections through qualified engineering review before installation.
Worked example data
| Input | Example value | Purpose |
|---|---|---|
| Bolt | M12 × 1.75, class 8.8 | Defines geometry and strength |
| Total external load | 50 kN | Applied joint tension |
| Bolt count | 2 | Shares total load |
| Load share C | 0.25 | External load entering each bolt |
| Preload target | 75% proof load | Sets clamp force |
| Torque coefficient | 0.20 | Estimates tightening torque |
Common bolt strength presets
| Preset | Proof (MPa) | Yield (MPa) | Ultimate (MPa) |
|---|---|---|---|
| Metric 4.6 | 225 | 240 | 400 |
| Metric 4.8 | 310 | 320 | 400 |
| Metric 5.8 | 380 | 400 | 500 |
| Metric 8.8 | 600 | 640 | 800 |
| Metric 9.8 | 650 | 720 | 900 |
| Metric 10.9 | 830 | 900 | 1,000 |
| Metric 12.9 | 970 | 1,080 | 1,200 |
| SAE Grade 2 | 393 | 393 | 510 |
| SAE Grade 5 | 586 | 635 | 827 |
| SAE Grade 8 | 827 | 896 | 1,034 |
| ASTM A307 Grade A | 248 | 248 | 414 |
| ASTM F3125 Grade A325 | 586 | 635 | 827 |
| ASTM F3125 Grade A490 | 827 | 896 | 1,034 |
| Stainless A2-70 | 450 | 450 | 700 |
| Stainless A4-80 | 600 | 600 | 800 |
Preset values are convenient defaults. Project specifications and certified material data govern.
Frequently asked questions
What is bolt tensile stress?
Bolt tensile stress is axial force divided by effective area. Threaded sections use tensile stress area. This area is smaller than nominal shank area.
Why use tensile stress area?
Threads reduce the effective resisting cross-section. Tensile area approximates this reduced section. It improves threaded bolt stress calculations significantly.
What is proof load?
Proof load limits permanent bolt deformation during testing. It often guides recommended preload. Certified grade data should confirm the value.
How is tightening torque estimated?
Torque uses the relationship T equals KFd. The coefficient represents friction effects. Actual preload can vary widely with friction.
What does joint stiffness ratio mean?
The ratio estimates external load entering the bolt. Stiffer bolts receive larger load fractions. Joint geometry controls both stiffness values.
Can this calculator check fatigue?
It provides a simplified Goodman fatigue estimate. Accurate fatigue needs reliable load histories. Surface, size, and notch effects also matter.
Does it verify thread stripping?
It estimates thread shear capacity using engagement length. The method remains intentionally simplified. Detailed thread standards should govern final verification.
Can one bolt carry uneven loading?
Yes, use distribution efficiency and critical-bolt factors. These increase the most-loaded bolt demand. Structural analysis should establish realistic values.
Is a safe result final approval?
No calculator replaces complete connection design. Shear, bearing, prying, fatigue, and separation matter. Obtain qualified review before field installation.
Engineering limitations and safety notice
This tool estimates idealized bolt tensile behavior. It does not replace applicable design standards. Material certificates and connection details remain essential.
Torque estimates are sensitive to thread friction. Actual preload may vary considerably during installation. Calibrated tensioning methods provide better preload control.
Check shear, bearing, prying, fatigue, and thread failure. Also verify temperature, corrosion, relaxation, and joint separation. Qualified engineers must approve safety-critical fastening designs before construction.