Bolt Torque Calculator
A free, browser-based calculator. Runs entirely in your browser — no sign up, nothing stored.
Enter Values
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.
Related tools
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.