Use this bolt tightening torque calculator to estimate the torque required to achieve a target bolt preload or clamp force. The calculator uses the simplified torque-preload relationship based on preload, bolt diameter and nut factor.
This calculator estimates the tightening torque required to produce a selected bolt preload. It uses the common torque-preload equation:
The calculator provides:
This calculator is useful for preliminary tightening torque estimates, clamp force planning and comparing different bolt diameters or nut factors.
Torque-preload relationship:
Where the units are:
If F is in newtons and d is in millimetres, then T is calculated in Nmm.
Torque conversion:
| Symbol | Description | Units |
|---|---|---|
| T | Tightening torque | Nmm or Nm |
| K | Nut factor | - |
| F | Target bolt preload / clamp force | N |
| d | Nominal bolt diameter | mm |
| Condition | Typical K Value |
|---|---|
| Well lubricated threads and bearing surfaces | 0.10 to 0.15 |
| Lightly lubricated or plated fasteners | 0.15 to 0.20 |
| Dry steel threads | 0.20 to 0.30 |
| Rough or inconsistent surfaces | May exceed 0.30 |
Nut factor values are approximate. Use measured or specified values where available.
Enter the target preload, bolt diameter and nut factor, then click calculate.
| Result | Meaning |
|---|---|
| Target preload | The desired clamp force in the bolt after tightening. |
| Nut factor | A simplified friction factor covering thread and bearing friction effects. |
| Tightening torque | The estimated torque required to achieve the selected preload. |
| Torque in Nm | The practical torque value normally used with torque wrenches. |
This calculator uses the simplified torque-preload equation and should be used as an approximate estimate only.
| This Calculator Includes | This Calculator Does Not Include |
|---|---|
| Target preload to torque calculation | Separate thread and under-head friction coefficients |
| Nut factor input | Torque scatter analysis |
| Nmm and Nm torque output | Embedment loss or preload relaxation |
| Worked calculation | Joint stiffness or external load sharing |
| Basic friction allowance through K | Fatigue, proof load or yield checks |
For estimating preload from an applied tightening torque, use the Bolt Preload Calculator.
For checking proof load and preload range, use the Bolt Proof Load Calculator.
For checking tensile stress in the bolt, use the Bolt Tensile Stress Calculator.
The simplified tightening torque equation relates the required torque to the target bolt preload, nominal bolt diameter and nut factor. The nut factor represents the combined effect of thread friction and bearing friction.
The nut factor is not a fixed material property. It can vary with lubrication, coating, washer condition, thread finish, surface condition and tightening method. This is why torque control alone can produce a wide scatter in actual bolt preload.
For critical bolted joints, use the relevant design standard, controlled tightening method, calibrated tools, direct tension indication or measured test data.
For further bolted joint theory, see the RoyMech reference page: Bolted Joint Design and Calculations.
For the full calculator section, see: Bolted Joint Calculators.
Tightening torque can be estimated using:
where T is tightening torque, K is nut factor, F is target preload and d is bolt diameter.
Nut factor is an empirical value used in the simplified torque-preload equation. It accounts for thread friction and bearing friction under the nut or bolt head.
Most tightening torque is lost to friction. Changes in lubrication, coating, washer condition, thread finish and surface condition can cause large variations in actual preload.
A value around 0.20 is often used for approximate calculations with dry or lightly lubricated steel fasteners, but the correct value depends on the actual fastener condition and test data.
No. This calculator estimates torque from a target preload. The bolt preload calculator estimates preload from an applied torque.
This calculator is intended for preliminary engineering calculations and educational use. Critical bolted joints should be checked using relevant standards, controlled tightening methods or test data.