Bolt Torque Calculator
Estimate bolt tightening torque from nominal diameter, target clamp force, and a friction (K) factor, using the short-form torque–tension relation. Metric and imperial, in your browser.
Tightening torque
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Show the math
The short-form torque–tension relation ties tightening torque to the clamp force it produces:
T = K × D × F
K is the nut factor (friction): lower for lubricated threads, higher when dry. These are planning estimates, for critical joints follow the fastener manufacturer's torque spec.
What this does
This calculator estimates the tightening torque for a bolt from its nominal diameter, the target clamp force, and a friction (K) factor, using the short-form torque–tension relation.
How to use it
- Enter the bolt’s nominal diameter.
- Enter the target clamp (preload) force.
- Pick a K factor for the thread condition.
- Read the estimated tightening torque.
How it works
The short-form relation uses K (nut/friction factor), D (nominal diameter), and F (target clamp force). It is a quick estimate, not a substitute for a manufacturer torque spec. Lubrication lowers K, so the same torque gives more clamp force.
T = K × D × F
Understanding your result
The torque shown is what delivers your target clamp force at the assumed friction factor, so lubrication matters: an oiled thread reaches the same clamp force at lower torque, and a dry one needs more. Use a manufacturer torque spec where one exists, and treat this as an estimate.
Example
An M10 bolt (D = 10 mm) at 20 kN clamp force with K = 0.2 needs about 40 N·m.
Sources & methodology
Last updated .
Frequently asked questions
How is bolt torque estimated?
Using the short-form relation T = K × D × F, where K is the nut/friction factor, D the nominal diameter, and F the target clamp (preload) force. It is a quick estimate, not a substitute for a manufacturer torque spec.
What K factor should I use?
K depends on the thread condition: roughly 0.20 for plain steel, about 0.22 zinc-plated, and around 0.15 when lubricated. Lubrication lowers friction, so the same torque gives more clamp force.
Is my data stored?
No. Everything is calculated locally in your browser.
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