Use this bolt proof load calculator to calculate bolt proof load from proof stress and tensile stress area. The calculator also estimates common preload ranges and checks an applied tensile or preload force against the calculated proof load.
This calculator determines the proof load of a bolt using proof stress and tensile stress area. Proof load is the load a bolt can withstand without permanent set under specified test conditions.
The calculator provides:
This calculator is useful for preliminary bolt selection, preload checks and comparing applied tensile load against a selected bolt proof load.
Bolt proof load:
Minimum recommended preload:
Maximum recommended preload:
Utilisation:
Factor of safety against proof load:
| Symbol | Description | Units |
|---|---|---|
| As | Tensile stress area | mm² |
| Sp | Proof stress | N/mm² |
| Fproof | Bolt proof load | N |
| F | Applied tensile or preload force | N |
| Pmin | Minimum preload percentage | % |
| Pmax | Maximum preload percentage | % |
| FoS | Factor of safety against proof load | - |
| Bolt Property Class | Typical Proof Stress |
|---|---|
| 4.6 | 225 N/mm² |
| 5.8 | 380 N/mm² |
| 8.8 | 600 N/mm² |
| 10.9 | 830 N/mm² |
| 12.9 | 970 N/mm² |
Always confirm values against the relevant fastener standard before final design.
Enter the tensile stress area and proof stress, then click calculate.
| Result | Meaning |
|---|---|
| Proof load | The calculated load corresponding to the entered proof stress and tensile stress area. |
| Recommended preload range | A preload estimate based on the entered percentages of proof load. |
| Applied load utilisation | The entered applied load divided by the calculated proof load. |
| Factor of safety | The calculated proof load divided by the entered applied load. |
| Pass/fail result | Pass means the entered applied load is below the calculated proof load. |
This calculator gives a simplified proof load and preload range estimate. It does not replace a full fastener design check.
| This Calculator Includes | This Calculator Does Not Include |
|---|---|
| Proof load from proof stress and stress area | Thread stripping |
| Preload range as percentage of proof load | Nut strength or tapped hole strength |
| Utilisation against proof load | Fatigue or fluctuating loads |
| Factor of safety against proof load | Prying action or joint stiffness |
| Worked calculation | Torque scatter or preload loss |
For calculating tensile stress area from thread diameter and pitch, use the Thread Tensile Stress Area Calculator.
For estimating preload from torque, use the Bolt Preload Calculator.
For checking axial tensile stress, use the Bolt Tensile Stress Calculator.
Bolt proof load is calculated by multiplying the tensile stress area by the proof stress. The proof stress is normally obtained from a fastener standard or material specification for the relevant bolt property class.
Proof load is often used when selecting an appropriate bolt preload. Preload is commonly specified as a percentage of proof load, but the correct value depends on the joint design, tightening method, friction conditions and design standard.
The tensile stress area should normally be used for threaded bolts where the thread is the critical section. Using nominal shank area may overestimate the proof load if the threaded portion governs.
For further screw thread data, see the RoyMech reference pages: Screw Stress Areas and Tensile and Yield Strength of Screws.
For the full calculator section, see: Bolted Joint Calculators.
Bolt proof load is the load a bolt can withstand without permanent set under specified proof test conditions. It is calculated from proof stress and tensile stress area.
Bolt proof load is calculated using:
where Sp is proof stress and As is tensile stress area.
Proof stress is a specified stress level used for fastener proof load testing. It depends on the bolt material, property class and applicable standard.
No. Proof load is a bolt strength-related value. Preload is the tensile force intentionally introduced when tightening a bolt.
Many preliminary calculations use a preload in the range of 60% to 75% of proof load, but the correct value depends on the joint design, tightening method and relevant standard.
No. This calculator calculates proof load and preload range. To estimate tightening torque from preload, use the Bolt Preload Calculator or Bolt Tightening Torque Calculator.