Bolt Torque Calculator
Torque Is a Proxy, Clamp Force Is the Real Goal
Nobody actually cares about torque for its own sake when tightening a bolted joint — what matters is the clamping force holding the joint together. Torque is simply the easiest thing to measure with a wrench, so the torque-tension relationship exists to translate a target clamp force into a torque value a technician can actually apply and verify.
The Formula
K is the nut factor (sometimes called the friction factor), which folds together thread friction, underhead friction, and thread geometry into a single empirical constant. It varies with surface finish and lubrication, which is why measured torque for a given clamp force can shift noticeably between a dry bolt and a lubricated one.
Worked Examples
| Diameter | Preload (Clamp Force) | K Factor | Required Torque |
|---|---|---|---|
| 10 mm | 20,000 N | 0.20 | 40.0 N·m |
| 6 mm | 8,000 N | 0.20 | 9.6 N·m |
| 12 mm | 30,000 N | 0.15 | 54.0 N·m |
K = 0.20 is a commonly used default for dry, non-plated steel fasteners; lubricated or plated fasteners typically use a lower K, which is why the same clamp force can call for noticeably less torque once a bolt is lubricated.
Where This Calculation Matters
- Setting a torque wrench spec — converting an engineering clamp-force requirement into the torque value written on an assembly drawing or work instruction.
- Preventing joint failure — under-torquing risks a joint loosening under vibration; over-torquing risks yielding the bolt or crushing the clamped material.
- Comparing fastener finishes — evaluating how a coating or lubricant changes the torque needed to hit the same clamp force.
How to Use This Calculator
- Enter Bolt Diameter (mm).
- Enter Preload / Clamp Force (N) — the target clamping force for the joint.
- Enter the Nut/Friction Factor K if known, or leave the default of 0.2.
- Select Calculate to get the required tightening torque.
Related Calculations
Once the joint is loaded, check the resulting stress with the Stress Strain Calculator.