Work out bolt torque instantly with clear inputs, formula shown and shareable results.
The short-form torque relation T = K x D x F bundles thread and head friction into a single nut factor: about 0.20 for plain steel, 0.15 lubricated and 0.28 for a zinc-plated fastener. Because friction consumes roughly 85 percent of the applied torque, the same torque can produce widely different preload if the surface condition changes.
Torque
T lb·in = K x D in x F lbf
Preliminary estimate only. Use the joint designer's or equipment manufacturer's specified torque; verify by a qualified engineer for safety-critical connections.
T lb·in = K x D in x F lbf. The short-form torque relation T = K x D x F bundles thread and head friction into a single nut factor: about 0.20 for plain steel, 0.15 lubricated and 0.28 for a zinc-plated fastener.
Only 10-15 percent of applied torque becomes preload; the rest overcomes friction. Scatter of plus or minus 25 percent in preload is normal, which is why critical joints use angle, stretch or ultrasonic control instead.
This calculator takes 3 inputs: Bolt nominal diameter, Target preload, Nut factor K. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.