Calculator inputs
Example data table
| Bolt | Torque (N·m) | K | Clamp force (kN) | Stress (MPa) | Utilization (%) |
|---|---|---|---|---|---|
| M8 × 1.25 | 22.00 | 0.20 | 13.75 | 375.60 | 58.69 |
| M10 × 1.50 | 45.00 | 0.20 | 22.50 | 388.00 | 60.63 |
| M12 × 1.75 | 80.00 | 0.20 | 33.33 | 395.57 | 61.81 |
| M16 × 2.00 | 180.00 | 0.18 | 62.50 | 398.93 | 48.06 |
Formula used
1) Clamp force from torque
F = T / (K × d)
F is clamp force, T is tightening torque, K is nut factor, and d is nominal diameter.
2) Tensile stress area
Metric: As = π/4 × (d − 0.9382p)²
Imperial: As = π/4 × (d − 0.9743/n)²
Use the optional custom area field when you already know the exact tensile stress area.
3) Stress, target preload, and reserve
σ = F / As
Ftarget = As × Sproof × preload%
Separation load = F / (1 − C)
C is the fraction of external load taken by the bolt. The remaining fraction reduces clamp force.
This method is widely used for quick design checks and shop-floor planning. For critical joints, verify with torque-angle, tension measurement, or fastener manufacturer data.
How to use this calculator
- Select metric or imperial units.
- Enter bolt diameter and either pitch or threads per inch.
- Type the applied torque and a realistic nut factor.
- Enter proof strength and your desired preload percentage.
- Add external tensile load and joint stiffness factor for reserve checks.
- Use the optional custom area when a standard table already gives the tensile stress area.
- Press calculate to show clamp force, stress, target torque, reserve, and the torque graph above the form.
- Use the CSV and PDF buttons to save the current result.
FAQs
1) What is bolt clamping force?
Bolt clamping force is the compressive force created when tightening stretches the bolt and squeezes the joined parts together. It controls joint grip, fatigue resistance, and slip behavior.
2) How accurate is the torque method?
It is a useful estimate, not a direct measurement. Real preload changes with lubrication, coatings, surface finish, washer behavior, thread condition, and wrench accuracy.
3) What does the nut factor K represent?
K bundles thread friction and bearing friction into one tightening coefficient. Lower friction lowers K and raises clamp force for the same torque.
4) Why compare preload with proof strength?
Proof strength shows how much tensile stress the fastener can sustain without permanent set. Comparing against it helps you avoid over-tightening while still reaching a useful preload.
5) Why include external load and joint stiffness factor?
External load does not go fully into the bolt. The stiffness factor estimates how the load splits between bolt stretch and joint compression loss.
6) What preload percentage is commonly used?
Many designs aim near 60% to 75% of proof load, but the best target depends on fatigue, sealing, gasket softness, relaxation, and installation control.
7) Should I use custom tensile stress area?
Use it when a standards table, fastener catalog, or thread class reference already gives the exact tensile stress area for your bolt and thread form.
8) Can this replace fastener testing?
No. It is best for screening, education, and early sizing. Critical assemblies still need validated specifications, tightening procedures, and sometimes direct preload verification.