Bolt Torque Calculator

Work out bolt torque instantly with clear inputs, formula shown and shareable results.

About Bolt Torque Calculator

The short-form torque relation T = K x D x F bundles thread and head friction into a single nut factor: about 0.20 for plain steel, 0.15 lubricated and 0.28 for a zinc-plated fastener. Because friction consumes roughly 85 percent of the applied torque, the same torque can produce widely different preload if the surface condition changes.

Formula

Torque

T lb·in = K x D in x F lbf

K: Nut factor, 0.15-0.28 by surface condition

Worked example

With the default values below, Bolt Torque Calculator returns Tightening torque of 66.7. Change any input above and every figure here is recalculated in your browser.

Inputs used

Bolt nominal diameterminimum 0.05 in: 0.5 in
Target preloadminimum 1 lbf: 8,000 lbf
Nut factor Kminimum 0.05: 0.2

Results

Tightening torque: 66.7 lb·ft
Torque in pound-inches: 800.0 lb·in
Torque in newton metres: 90.4 N·m
Clamp force: 35.59 kN

Step by step

1Values used
2Torque
3Tightening torque= 66.7 lb·ft
4Torque in pound-inches= 800.0 lb·in
5Torque in newton metres= 90.4 N·m
6Clamp force= 35.59 kN

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Preliminary estimate only. Use the joint designer's or equipment manufacturer's specified torque; verify by a qualified engineer for safety-critical connections.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Use the joint designer's or equipment manufacturer's specified torque; verify by a qualified engineer for safety-critical connections.

Frequently Asked Questions

How is the Bolt Torque figure worked out?

T lb·in = K x D in x F lbf. The short-form torque relation T = K x D x F bundles thread and head friction into a single nut factor: about 0.20 for plain steel, 0.15 lubricated and 0.28 for a zinc-plated fastener.

Why is torque control so imprecise?

Only 10-15 percent of applied torque becomes preload; the rest overcomes friction. Scatter of plus or minus 25 percent in preload is normal, which is why critical joints use angle, stretch or ultrasonic control instead.

What values do I need to enter?

This calculator takes 3 inputs: Bolt nominal diameter, Target preload, Nut factor K. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.