Bolt Preload Calculation
A bolt preload calculation converts tightening torque into clamp force. The key input is nut factor K, because friction changes how much of the torque becomes bolt tension and how wide the preload scatter can be.
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Step-by-step calculation
- Write the applied torque T.
- Choose nut factor K for dry, lubricated or coated threads.
- Convert nominal diameter d to metres when torque is in N*m.
- Calculate F = T / (K * d).
- Compare F with proof load from thread stress area and proof strength.
Dry M10 example
For an M10 bolt at 30 N*m dry with K = 0.20: F = 30 / (0.20 * 0.010) = 15,000 N.
Lubricated M10 example
Keep torque at 30 N*m but change to K = 0.15: F = 20,000 N. The same torque produces about one-third more preload.
Related pages
- Bolt preload torque calculator - calculate clamp force, proof-load utilization and preload scatter.
- Bolt preload formula - equation and nut-factor definitions.
- Bolt preload explained - joint mechanics, proof load targets and preload scatter.
- Bolt torque spec chart - starting torque values by size and grade.