Bolt Torque Calculator

T = K × D × F, where K is the nut factor (≈0.20 dry, ≈0.15 lubricated), D is nominal diameter, and F is the target preload (clamp force). Lubrication lowers K, so the same torque yields more preload.

Commonly 75% of the bolt's proof load for reusable joints. Proof load depends on grade (e.g., Grade 5/8, class 8.8/10.9) and the tensile-stress area. This tool is an estimate - follow the engineering spec for critical joints.

Torque mostly fights friction, not stretch - only ~10-15% of torque becomes preload. Small changes in lubrication or surface finish (K) swing preload widely, which is why torque control is imprecise.

No. Torque-only is ±25-35% on preload. Turn-of-nut, torque-to-yield, or bolt-tension measurement are more accurate for critical fasteners.

Preload must not exceed the bolt's proof load (roughly 90% of yield). Going beyond it can cause permanent deformation or failure. Always check strength against the bolt grade (8.8, 10.9, A325, A490).

Breaking torque is the minimum torque that snaps the bolt; tightening torque is the service torque that produces the target preload, usually far below the breaking torque. The ratio between the two sets the safety margin of the bolted joint.

Use the long-form model (explicit pitch, thread half-angle, and separate friction coefficients) or direct measurement methods (ultrasonic, bolt stretch) when preload uncertainty must stay below ±10%, such as certified structural connections, pressure flange joints, or high-precision engine assemblies.