Bolt Torque Calculator
Estimate the tightening torque needed for a target bolt clamp force.
Inputs
≈0.2 for dry steel-on-steel; lubricated threads can be as low as 0.10-0.15.
Saved Scenarios
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Required Torque (Nm)
40.00
Spark says
How it's calculated
Formula
- K
- — Friction coefficient (nut factor), depends on lubrication and surface finish
- D
- — Bolt nominal diameter
- F
- — Target clamp (preload) force
What is the Bolt Torque Calculator?
This is the widely-used short-form torque-to-clamp-force relationship — an approximation, since only about 10% of tightening torque actually creates clamp force (the rest overcomes thread and under-head friction), but it's accurate enough for general fastening work.
How to use it
- 1 Enter the bolt's nominal diameter.
- 2 Enter the clamp force you're targeting (from a bolt strength calculation or manufacturer spec).
- 3 Enter a friction coefficient (K-factor) appropriate for the surface condition and lubrication.
Worked examples
Limitations
- •This is an approximation with real-world scatter of ±25% or more — for safety-critical or precision joints, use manufacturer torque specs or a calibrated tension method instead.
Frequently asked questions
Why does lubrication reduce required torque?
Lubrication reduces friction under the bolt head and in the threads — since most torque fights friction rather than creating clamp force, a lower K-factor means less torque achieves the same clamp force.