How do you calculate the required tightening torque for a bolt?

Tightening torque T = K x D x Fi, where K is torque coefficient (0.2 for dry steel, 0.15 for lubricated), D is nominal bolt diameter, and Fi is desired preload. Preload is typically 75-90% of bolt proof load for reusable connections. For M12 Grade 8.8 bolt (proof load ~48kN) with 80% utilization and lubricated threads: T = 0.15 x 0.012 x 38,400 = 69 Nm. Always verify with torque-angle method for critical joints.