Bolt Tightening Torque Calculator

The torque coefficient method

The simplified torque-coefficient (nut factor) method relates tightening torque to bolt preload using T = K × D × F, where T is torque, K is the torque coefficient, D is the bolt nominal diameter and F is the target preload. Around 90% of applied torque is typically consumed overcoming friction under the bolt head or nut and in the threads, with only a small proportion converted into useful clamp force.

The torque coefficient K depends heavily on friction condition, including lubrication, plating, surface finish and whether the fastener has been reused. Because K can vary considerably even for nominally identical bolts, this method gives an approximate torque only. Where accurate preload control is critical, direct tension measurement, torque-angle tightening or a calibrated torque wrench with a verified K value for the actual fastener condition should be used instead.