Example inputs
Click one example to auto-fill the form, then press Calculate.
| Example | Mode | Bolt | Key inputs |
|---|---|---|---|
| 1 | Torque → preload | M10×1.5 | K=0.20, T=60 N·m, External=5,000 N, C=0.25 |
| 2 | Stress → preload | 1/2-13 UNC | Stress=85,000 psi, External=2,000 lbf, C=0.25 |
| 3 | Preload → torque | Custom d=12 mm | F=20 kN, K=0.18, External=0, C=0.25 |
Example data table
| Case | Size | K | Torque | Estimated preload | External load | Clamp after load |
|---|---|---|---|---|---|---|
| Torque method | M10×1.5 | 0.20 | 60 N·m | ≈ 30,000 N | 5,000 N | ≈ 26,250 N (C=0.25) |
| Stress method | 1/2-13 UNC | 0.20 | (derived) | ≈ 12,000 lbf | 2,000 lbf | ≈ 10,500 lbf (C=0.25) |
| Target force | Custom d=12 mm | 0.18 | (computed) | 20 kN | 0 | 20 kN |
Formula used
- F = T / (K·d) for torque-based preload.
- F = σ · At for stress-based preload.
- T = K·F·d for torque required to reach a target preload.
- Fclamp = Fpreload − (1 − C)·P for remaining clamp under external separating load.
Here K is the nut factor, d is nominal diameter, At is tensile stress area, σ is bolt stress, P is external load, and C is the bolt load fraction.
How to use this calculator
- Select a calculation mode that matches your known values.
- Choose the bolt size, or enter a custom diameter.
- Enter nut factor K based on lubrication and finish.
- Provide torque, target stress, or desired preload as needed.
- Optionally enter external separating load and bolt fraction C.
- Click Calculate to review preload, torque, and clamp margin.
- Use CSV or PDF to save results for documentation.
Tip: If you know the exact tensile stress area from a datasheet, enable custom area for best accuracy.
Bolt clamping force essentials
1) What clamping force really means
Clamping force (preload) is the tension created in a bolt during tightening. That bolt tension compresses the joint parts and generates friction that resists slip and loosening. In many mechanical joints, designers aim for a preload near 70–80% of the fastener proof load, assuming the joint materials and contact surfaces can safely carry the compression.
2) Torque-to-preload relationship (and its limits)
Torque is a convenient input, but it is an indirect predictor of preload because friction consumes most of the tightening energy. A widely used estimate is F ≈ T / (K·d), where T is torque, d is nominal diameter, and K is the nut factor. Typical K values are 0.10–0.13 (well-lubricated), about 0.18 (lightly oiled), and 0.20–0.25 (dry or rough). Using the wrong K can shift preload by tens of percent.
3) Stress and tensile stress area method
When allowable bolt stress is known, preload can be estimated with F = σ·A. Here σ is stress in MPa and A is tensile stress area in mm², producing force in newtons. For metric coarse threads, approximate tensile areas are: M8 ≈ 36.6, M10 ≈ 58.0, M12 ≈ 84.3, M16 ≈ 157, and M20 ≈ 245 mm². If your standard or datasheet lists a different area, enter it as a custom value for best accuracy.
4) External separating load and clamp margin
Real assemblies often see separating loads from pressure, vibration, bending, or thermal effects. Only a fraction of that external load increases bolt tension; the remainder reduces joint compression. This calculator uses a stiffness split ΔFbolt = C·Fext. Stiff joints commonly fall around C = 0.2–0.3, while softer joints can be higher. The remaining clamp margin helps you judge whether the joint is likely to separate under the applied external load.
5) Practical input checks and tightening guidance
Keep units consistent: torque in N·m, diameter in mm, stress in MPa, and results in kN. Small diameter or unit mistakes create large force errors because preload scales with 1/d. For critical joints, torque-only control may be insufficient; consider torque-angle, direct tension indicators, or measurement-based tightening. Reuse, embedding, and coatings can change friction and reduce achieved preload, so always validate results against fastener grade limits and approved procedures.
FAQs
What is a typical nut factor K value?
K depends on friction. Well-lubricated fasteners may be 0.10–0.13, lightly oiled around 0.18, and dry or rough surfaces commonly 0.20–0.25. Use test data or a specification when possible.
Why does the same torque give different clamp forces?
Because friction varies with lubrication, plating, surface roughness, and thread condition. Most of the input torque is consumed by friction, so small friction changes create large preload changes.
Should I target 100% of proof load?
No. Many designs target about 70–80% of proof load to reduce loosening while keeping a margin against yielding. The right target depends on joint stiffness, vibration, and materials.
What does the external load fraction C represent?
C is the portion of an external separating load that increases bolt tension. Stiff joints often have lower C (around 0.2–0.3). Softer joints or long grip lengths can raise C.
When should I use the stress method instead of torque?
Use the stress method when you have a defined allowable stress and a reliable tensile stress area from a standard or datasheet. It avoids friction uncertainty but still needs grade limits for safety.
Is this calculator suitable for flange or gasket joints?
It can provide a first estimate, but gasketed flanges require additional checks for gasket seating stress, relaxation, and bolt pattern effects. Follow applicable standards and manufacturer guidance for final sizing.