Bolt Torque and Preload Calculator
Compute the assembly torque for a target bolt preload: tensile stress area from thread geometry, yield load from the property class, and torque T = K x F x D with a lumped friction factor for dry, oiled or plated threads.
How to use this calculator
- Enter nominal diameter and pitch, or take them from the bolt marking.
- Select the property class (4.8, 5.8, 8.8, 10.9, 12.9) and thread condition.
- Set the preload as a percentage of yield, typically 75 percent for machine design.
Frequently asked questions
What torque should I use for an M12 8.8 bolt?
For a dry M12 x 1.75 class 8.8 bolt at 75 percent yield preload, the torque is roughly 95 to 100 N·m. Lubricated threads need less torque for the same preload, about 70 to 75 N·m.
Why does friction dominate the torque?
About 50 percent of the input torque goes into thread friction and 40 percent into bearing friction under the head. Only roughly 10 percent stretches the bolt, which is why friction condition changes torque so strongly.
Is 75 percent of yield always right?
It is a common static joint default. Dynamic loads, gasketed joints and thermal cycling usually call for 60 to 70 percent, and torque-controlled tightening has 25 to 40 percent scatter that must be accounted for.