Bolt Torque Calculator
Proper bolt torque is essential for safe, reliable connections in construction, automotive, and mechanical applications. Under-torquing leads to loose joints and vibration failure; over-torquing causes bolt stretch, thread stripping, or breakage. This calculator provides recommended torque values based on bolt diameter, grade, and lubrication condition.
Bolt torque is calculated using the formula T = K × F × d, where T is torque, K is the nut factor (0.20 for dry, 0.15 for lubricated), F is the desired clamp force, and d is the nominal bolt diameter. The clamp force target is typically 75% of the bolt's proof load, providing a safety margin while generating adequate clamping pressure.
Bolt grade determines tensile and proof load strength. Grade 2 bolts (no markings on head) are basic low-carbon steel for non-critical applications. Grade 5 (3 radial lines on head) are quenched and tempered medium-carbon steel suitable for most structural connections. Grade 8 (6 radial lines) are high-strength alloy steel for demanding applications like suspension and engine assemblies.
Lubrication dramatically affects torque. A lubricated bolt generates the same clamp force at roughly 75% of the dry torque value. Always specify whether the torque spec assumes dry or lubricated conditions. Never apply oil or anti-seize to bolts unless the torque specification calls for lubricated values, as this can easily result in over-tightening and bolt failure.