Use this bolt preload calculator to estimate bolt clamp force from tightening torque, bolt diameter and nut factor. The calculator uses the common torque-preload relationship to estimate preload and compare it with a selected proof load or allowable bolt load.
This calculator estimates the preload, or clamp force, generated in a bolt when a tightening torque is applied. It uses the simplified torque-preload equation:
The calculator provides:
This calculator is useful for preliminary bolt tightening estimates, clamp force checks and comparing different nut factors or bolt diameters.
Torque-preload relationship:
Estimated bolt preload:
Design allowable preload:
Utilisation:
Factor of safety:
| Symbol | Description | Units |
|---|---|---|
| T | Tightening torque | Nmm |
| K | Nut factor | - |
| F | Estimated bolt preload / clamp force | N |
| d | Nominal bolt diameter | mm |
| Fallow | Allowable bolt load or proof load | N |
| SF | Safety factor | - |
| Fdesign | Design allowable preload | N |
| Condition | Typical K Value |
|---|---|
| Well lubricated threads | 0.10 to 0.15 |
| Lightly lubricated / plated fasteners | 0.15 to 0.20 |
| Dry steel threads | 0.20 to 0.30 |
| Rough or inconsistent surfaces | May exceed 0.30 |
Enter the tightening torque, bolt diameter and nut factor, then click calculate.
| Result | Meaning |
|---|---|
| Estimated preload | The approximate clamp force generated by the applied tightening torque. |
| Design allowable preload | The entered allowable bolt load divided by the selected safety factor. |
| Utilisation | The estimated preload divided by the design allowable preload. A value below 100% passes this simplified check. |
| Factor of safety | The entered allowable bolt load divided by the estimated preload. |
| Nut factor | A simplified friction factor that accounts for thread and bearing friction effects. |
This calculator uses a simplified torque-preload equation. It is useful for quick estimates but should not be treated as a precise preload prediction.
| This Calculator Includes | This Calculator Does Not Include |
|---|---|
| Torque to preload estimate | Detailed thread friction calculation |
| Nut factor input | Separate thread and bearing friction coefficients |
| Torque unit conversion | Bolt relaxation or embedment loss |
| Utilisation against entered allowable load | Joint stiffness or external load sharing |
| Worked calculation | Fatigue, proof testing or tightening scatter analysis |
For combined tensile and shear stress in a bolt, use the Bolt Combined Stress Calculator.
For bolt shear stress only, use the Bolt Shear Stress Calculator.
The simplified torque-preload equation is widely used for approximate bolt tightening calculations. It relates tightening torque to bolt preload using a nut factor, which represents the combined effects of thread friction and bearing friction.
The nut factor is not a fixed material property. It can change significantly with lubrication, coating, washer condition, thread finish, tightening speed and surface condition. For critical bolted joints, controlled tightening procedures or direct preload measurement may be required.
The calculated preload should be checked against an appropriate bolt proof load, allowable preload or design limit. The value entered into this calculator should be selected by the engineer based on bolt grade, thread size, design standard and joint requirements.
For further bolted joint theory, see the RoyMech reference page: Bolted Joint Design and Calculations.
For the full calculator section, see: Bolted Joint Calculators.
Bolt preload can be estimated using the simplified equation:
where T is tightening torque, K is the nut factor and d is the bolt diameter.
Bolt preload is the tensile force developed in a bolt when it is tightened. This force clamps the connected parts together.
The nut factor is an empirical value used in the simplified torque-preload equation. It accounts for friction in the threads and under the nut or bolt head.
Most tightening torque is lost to friction. Small changes in thread friction, lubrication, coating or surface condition can cause large changes in actual bolt preload.
A nut factor around 0.20 is often used for approximate calculations with dry or lightly lubricated steel fasteners, but the correct value depends on lubrication, coating, surface condition and test data.
This calculator is intended for preliminary engineering calculations and educational use. Critical bolted joints should be checked using relevant design standards, controlled tightening methods or test data.