Bolt Torque Calculator
Computes bolt tightening torque from target preload, diameter and nut factor.
Enter Values
Minimum 0
Minimum 0
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the bolt's nominal diameter (mm) and the target preload/clamp force (kN).
- Adjust the nut factor K for the thread condition (dry, lubricated, plated).
- Read the tightening torque.
How it works
The Bolt Torque Calculator uses the short-form torque–tension rule T = K · D · F, where D is the bolt diameter, F the desired preload and K the nut factor capturing thread and head friction.
K is roughly 0.20 dry/as-received, 0.16 lightly lubricated and 0.22 zinc-plated. Most of the torque overcomes friction, so K matters a lot.
Worked example
M12, 25 kN preload, K = 0.2. T = 0.2 × 25,000 N × 0.012 m = 60 N·m to develop the target clamp force.
Tips
- If a torque spec exists for your fastener or joint, use it instead of this estimate.
Frequently asked questions
How accurate is the torque–tension rule?
It's an estimate — actual preload from a set torque can vary ±25–35% because friction varies. Where preload is critical, use a calibrated method (angle/turn-of-nut, tension indicators) and always follow the manufacturer's or joint's torque spec.
Can I use these results for final design?
No. These calculators are for first-pass sizing, checking and learning. They assume ideal supports, static loads and elastic behaviour, and they don't apply load factors, member capacity, buckling or connection checks. Always verify against the relevant design code (e.g. AS 4100, AS 1170, Eurocode) and have a qualified engineer sign off structural work.
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Cite this tool
Zerdly. (2026). Bolt Torque Calculator. Retrieved from https://www.zerdly.com/tools/bolt-torque-calculator/
Tip: Enter any known values to calculate the remaining results.
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