# Bolt Torque Calculator Calculate recommended bolt torque values for Grade 2, 5, 8, and stainless steel fasteners. Get dry and lubricated torque specs, clamp force, and proper. ## What this calculates 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. ## Inputs - **Bolt Diameter** — options: 1/4", 5/16", 3/8", 1/2", 5/8", 3/4", 1" — Nominal bolt diameter - **Bolt Grade** — options: Grade 2 (low carbon steel), Grade 5 (medium carbon, quenched), Grade 8 (alloy, quenched & tempered), 18-8 Stainless Steel — Higher grades have greater tensile strength - **Lubricated** — Oil, anti-seize, or wax coating on threads ## Outputs - **Dry Torque** (ft-lbs) — Recommended torque for dry (unlubricated) bolts - **Lubricated Torque** (ft-lbs) — Recommended torque for lubricated bolts - **Clamp Force** (lbs) — Axial clamping force generated at recommended torque - **Recommended Torque** (ft-lbs) — Based on your dry/lubricated selection ## Details 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. ## Frequently Asked Questions **Q: What do the lines on a bolt head mean?** A: The radial lines on a hex bolt head indicate its grade. No lines = Grade 2. Three lines = Grade 5. Six lines = Grade 8. Stainless steel bolts are usually marked with the material type (e.g., 18-8 or A2). Never substitute a lower grade bolt for a higher grade one in structural applications. **Q: Should I torque bolts dry or lubricated?** A: Use dry torque values unless the specification explicitly calls for lubricated bolts. Lubrication reduces friction by about 25%, meaning the same torque generates significantly more clamp force. If you apply oil or anti-seize to threads without reducing the torque value, you can over-stress or break the bolt. **Q: What is the difference between torque and clamp force?** A: Torque is the rotational force you apply with a wrench (measured in ft-lbs). Clamp force is the axial tension in the bolt that holds the joint together (measured in lbs). About 90% of the torque you apply goes to overcoming friction; only 10% generates actual clamp force. This is why lubrication matters so much. **Q: Why does my bolt break when I torque it?** A: Common causes: exceeding the recommended torque, using a lower-grade bolt than specified, re-using a bolt that has already been stretched, cross-threading (which increases friction dramatically), or using lubricated torque values on a dry bolt specification. Always verify bolt grade and follow the correct dry/lubricated spec. --- Source: https://vastcalc.com/calculators/construction/bolt-torque Category: Construction Last updated: 2026-04-21