Bolt Torque Specification Calculator

Correct bolt torque is critical in construction and mechanical assembly. Under-torquing allows joints to loosen over time, especially under vibration. Over-torquing stretches the bolt beyond its yield point, permanently deforming it and weakening the connection — or snapping the bolt entirely.

How Torque Is Used to Create Clamping Force

When you tighten a bolt, the torque applied converts to two things: friction (about 50% for dry conditions) and clamping force (stretching the bolt like a spring). The bolt’s tension — called preload — is what holds the joint together.

The approximate relationship is: T = K × D × F

Where:

• T = Torque (N·m or ft·lb)
• K = Torque coefficient (≈ 0.2 for dry steel, ≈ 0.15 for lubricated)
• D = Bolt diameter (m or in)
• F = Target preload force (N or lb)

Bolt Grade Guide

Metric bolts:

• Grade 4.6: Low strength, general purpose
• Grade 8.8: Medium-high strength (most common structural)
• Grade 10.9: High strength (e.g., flange bolts)
• Grade 12.9: Very high strength (aerospace, critical)

Imperial/SAE bolts:

• Grade 2: Low strength (no markings on head)
• Grade 5: Medium strength (3 marks on head)
• Grade 8: High strength (6 marks on head)

Standard Torque Values (Metric, Grade 8.8, Dry)

Important Notes

Always torque bolts in a crisscross pattern when tightening multiple bolts in a pattern (like a cylinder head or flange). Apply torque gradually in 2–3 stages. Lubricated bolts require about 25% less torque than the dry specification. Never reuse single-use bolts (stretch bolts used in engines and critical assemblies).