Calculator
Formula Used
The calculator uses the common torque-preload relationship:
F = T / (K × D)
Where F is bolt tension, T is torque, K is the nut factor, and D is nominal bolt diameter.
When tensile stress area is supplied, stress is calculated as:
Stress = F / A
When proof strength is supplied, proof load is calculated as:
Proof Load = Proof Strength × Tensile Stress Area
How to Use This Calculator
- Enter the applied torque and select its unit.
- Enter the bolt diameter and choose millimeters or inches.
- Select an assembly condition or enter your own nut factor.
- Add tensile stress area for stress calculations.
- Add proof strength to check proof load usage.
- Enter a target proof percentage to estimate target torque.
- Press Calculate to show the result below the header.
- Use CSV or PDF buttons to save the result.
Example Data Table
| Torque | Diameter | K Factor | Estimated Tension | Typical Use |
|---|---|---|---|---|
| 80 N·m | 10 mm | 0.20 | 40,000 N | Dry joint estimate |
| 100 N·m | 12 mm | 0.18 | 46,296 N | Plated fastener estimate |
| 120 N·m | 12 mm | 0.15 | 66,667 N | Lightly lubricated estimate |
| 150 N·m | 16 mm | 0.20 | 46,875 N | Larger bolt comparison |
Torque to Tension Guide
A torque to tension calculator helps turn a wrench setting into estimated bolt preload. It is useful when drawings, manuals, or field notes give torque, but the work needs clamp force. The method is simple, yet the result depends heavily on friction. Threads, washers, coatings, plating, surface finish, and lubricant can all change the real preload.
Why Bolt Tension Matters
Bolt tension creates the clamping force that holds parts together. Too little tension can let joints slip, leak, loosen, or fatigue. Too much tension can yield the fastener, crush parts, or damage threads. A practical estimate gives technicians a starting point before using direct tension tools, ultrasonic measurement, or load indicating hardware.
Understanding the Nut Factor
The nut factor, often called K, combines thread friction, bearing friction, and thread geometry into one value. Dry steel bolts may use a higher value. Lubricated bolts often use a lower value. Small K changes can create large preload changes, so always use project data when available.
Useful Results
This tool returns tension in newtons, kilonewtons, pounds force, and kilogram force. It also estimates stress when tensile stress area is supplied. If proof strength is entered, the page compares preload with proof load and displays a proof safety factor. The target preload feature also shows the torque needed for a selected proof percentage.
Good Engineering Practice
Treat torque based tension as an estimate, not a guarantee. Calibrate torque tools often. Clean threads before assembly. Use the same lubricant assumed by the calculation. Tighten critical patterns in stages. Record actual values for traceability.
When to Use Extra Checks
Use extra verification when joints are safety critical, highly loaded, hot, vibrating, or sealed against pressure. In those cases, torque alone may be weak evidence. Add joint testing, inspection marks, tightening sequences, or measured elongation. A careful plan reduces rework and improves reliability.
Reading the Output
Compare the calculated tension with the joint goal. A value near proof load may be risky unless the specification requires it. A very low value may not seal or resist shear. Use the exported record to share assumptions, especially diameter, units, K value, area, and proof strength. Clear records make later reviews easier during maintenance and audits.
FAQs
What is a torque to tension calculator?
It estimates bolt preload from applied torque, bolt diameter, and nut factor. It helps compare clamp force, proof load usage, and tightening assumptions before assembly.
What is nut factor K?
Nut factor K is an empirical value for friction and geometry. It combines thread friction, bearing friction, and fastener behavior into one practical coefficient.
Is torque preload exact?
No. Torque preload is an estimate. Surface condition, lubrication, thread quality, washers, coatings, and tool accuracy can change actual bolt tension significantly.
Which K value should I use?
Use the value from your specification, test data, or fastener supplier. If none exists, use presets only as rough starting estimates.
Why enter tensile stress area?
Tensile stress area lets the calculator estimate bolt stress. It is needed when you want stress, proof load comparison, or proof safety factor.
What is proof load usage?
Proof load usage compares estimated preload with the bolt proof load. Higher percentages may be acceptable in some designs but require careful specification.
Can I use inches and foot-pounds?
Yes. The calculator accepts inch diameter, foot-pound torque, inch-pound torque, and common strength units. It converts values internally.
Should critical joints use this only?
No. Critical joints should use approved procedures, calibrated tools, inspection, and direct verification when required. This calculator supports planning only.