Bolt torque calculator
The common short-form relation between torque and preload. Enter the target preload, bolt diameter and a nut factor that matches the lubrication.
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How to use it
- Enter nominal bolt diameter, target preload and nut factor k.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
T = K × F × d, where K is the nut factor, F the preload and d the nominal diameter. 1 lbf·ft = 1.3558 N·m.
Example
Inputs: Nominal bolt diameter = 12 mm, Target preload = 20 kN, Nut factor K = 0.2. Result: Tightening torque: 48 N·m; Torque in lbf·ft: 35.4 lbf·ft.
Limits and caveats
- An estimate only: friction scatter means real preload can differ widely from the target.
- Follow the maker or standard torque when one is given, especially on engines and critical joints.
Questions about this tool
What value of K should I use?
A common value for dry, unlubricated steel is about 0.2; lubrication lowers it to about 0.15 or less depending on the materials.
What preload should I target?
Many references suggest about 75% of the bolt proof load for reusable joints. Proof load depends on the grade and size.
Why not tighten as hard as possible?
Over-tightening can pass yield, so the bolt stretches, snaps or strips its threads.