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 past its yield point, permanently deforming it and weakening the connection, or snapping it outright.

How Torque Is Used to Create Clamping Force

When you tighten a bolt, the torque applied converts to two things: friction (about 50% of it under the head and in the threads when the bolt is dry) and clamping force, which stretches the bolt like a spring.
That tension is called preload, and it is what actually holds the joint together. Torque is only the means of producing it.

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

Where:

• T = Torque (N·m or ft·lb)
• K = Torque coefficient (about 0.20 for dry steel, 0.15 lubricated, 0.13 with anti-seize)
• D = Bolt diameter (m or in)
• F = Target preload force (N or lb)

This calculator runs that formula backwards and shows you the preload each torque figure buys, which is the number that actually matters.
It is also why lubricating a bolt is not “cheating”: a lubricated bolt gets the same clamping force from less torque, because less of your effort is being burned as friction. The three lubrication settings here reduce the torque in exactly the ratio the K values drop, so the preload comes out identical on all three. That is the point.

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)

Every preload figure above lands within about 5% of 75% of the ISO 898-1 proof load for a Grade 8.8 bolt that size (proof stress 580 MPa on the tensile stress area). That is where a general-purpose torque table should sit: enough clamp to keep the joint closed, with room left before the bolt starts to stretch for good.

Important Notes

Always tighten a group of bolts in a crisscross pattern, the way a cylinder head or a flange is done. Work up to the final figure in three passes at roughly 30%, 70% and 100%, so the joint pulls down evenly instead of cocking to one side.
Lubricated bolts need about 25% less torque than the dry specification, and anti-seize about 35% less. Never reuse a stretch bolt (the single-use torque-to-yield type used in engines and other critical assemblies), because it has already given up most of its elastic range.

One habit worth having: if a torque figure feels wrong for the size of the fastener in your hand, stop and check the spec rather than trusting the wrench. Torque tables assume clean, undamaged threads, and a galled thread can absorb most of the reading before the bolt sees any tension at all.