Bolt Torque Calculator

Calculate tightening torque and clamp load for a bolted joint.

Please note: These are idealised textbook models using representative material data. They are for study, sizing and sanity-checking only — never for final design. Real engineering requires code-compliant analysis, verified material certificates and a qualified engineer’s sign-off.

What the Bolt Torque Calculator does

Tightening a bolt stretches it, and that stretch is the clamp load holding the joint together. Torque is only an indirect way of reaching it — most of the effort goes into overcoming friction, which is why the nut factor and the thread condition matter as much as the bolt itself.

Formula

T = K × D × F
F = Proof strength × Stress area × Target share
Proof load = Stress area × Proof strength
K ≈ 0.20 dry, 0.15 lubricated

Inputs explained

Input	Unit	Required	Notes
Bolt size	one of 8 options	Yes	—
Property class	one of 4 options	Yes	—
Thread condition	one of 5 options	Yes	—
Preload as a share of proof load	%	Yes	Typical practice is 75% for reusable joints, up to 90% for permanent ones.
Custom nut factor	number	Optional	Overrides the thread condition if set. Accepts 0 or more.

How to use it

Choose Bolt size and Property class.
Enter Preload as a share of proof load.
Optionally add Custom nut factor.
Select Calculate.

Worked example

An M12 class 8.8 bolt, dry threads, tightened to 75% of proof load.

Bolt: M12 × 1.75
Class: 8.8
Condition: Dry
Preload: 75%

Proof load = 84.3 mm² × 600 MPa = 50.6 kN, so preload is 37.9 kN. T = 0.20 × 0.012 × 37,935 = 91.0 N·m.

Frequently asked questions

Why does lubrication change the torque so much?

Because most of the torque fights friction, not tension. Lubricating drops K from about 0.20 to 0.15, so the same torque produces roughly a third more clamp load — enough to snap the bolt.

How accurate is torque as a way of setting preload?

Poor — typically ±25% even with care. Where preload is critical, measure bolt stretch or use load-indicating hardware instead.

Method and sources

Method. The short-form torque relationship T = K × D × F, where K is the nut factor, D the nominal diameter and F the target preload taken as a share of proof load.

Assumptions

The nut factor entered represents the actual friction condition of the threads and bearing face.

Limitations

The nut factor is an empirical lumped constant, not a property. Published values around 0.20 dry and 0.15 lubricated are approximations, and real values scatter widely with surface finish, plating, lubricant and reuse.
Because most applied torque overcomes friction rather than stretching the bolt, torque control typically achieves preload only within roughly ±25–30%. Critical joints use angle control, bolt elongation or direct tension measurement for this reason.
Nothing here establishes that a joint design is adequate — that is a matter of joint stiffness, external load, fatigue and the governing code.

Sources

The fastener manufacturer's torque specification, and the code governing the joint — Varies by manufacturer and jurisdiction. The specified torque for a real assembly. A general nut-factor calculation is a starting estimate, and a published specification supersedes it.