What is the nut factor K?

The nut factor (also called the torque coefficient) accounts for friction between the bolt threads and the nut or mating surface. Typical values: dry steel ~0.20, lightly oiled ~0.15, zinc-plated ~0.17, waxed or MoS2-coated ~0.13. Always use the value matching your actual lubrication condition.

What units does this calculator use?

You can enter diameter in millimeters or inches and clamp force in kilonewtons or kips. The tool converts everything internally and outputs torque in both Newton-meters (N·m) and foot-pounds (ft·lb).

Is this formula exact?

T = K × D × F is an engineering approximation widely used in industry. It does not account for thread pitch, bearing face diameter, or variations in surface finish. For critical safety applications, always consult a structural engineer and follow relevant standards (AISC, VDI 2230, etc.).

What if I do not know the clamp force?

Clamp force is typically 70–75% of the bolt's proof load. You can find proof load in bolt manufacturer data sheets or ASTM/ISO bolt-grade tables. Multiply proof load by 0.70–0.75 to get the target clamp force.

Can I use this for metric and imperial bolts?

Yes. Select "mm" and "kN" for metric bolts (M8, M10, M12, etc.) or "in" and "kips" for imperial bolts (1/4″, 3/8″, 1/2″, etc.). The result is shown in both N·m and ft·lb regardless of the input unit choice.

Bolt Torque Calculator

Name: Bolt Torque Calculator
Availability: InStock
Author: Nham Vu

Enter bolt diameter, clamp force, and torque coefficient to instantly calculate required tightening torque.

Bolt Parameters

Nominal Bolt Diameter

Target Clamp Force

Nut Factor K (torque coefficient)

Quick presets:

Result

Fill in the parameters and click Calculate.

Summary

Enter bolt diameter, clamp force, and torque coefficient to instantly calculate required tightening torque.

How it works

Enter the nominal bolt diameter in millimeters or inches.
Enter the required clamp force in kilonewtons or kips.
Select a nut factor (K) preset or type a custom value.
Click "Calculate" to apply T = K × D × F.
Read the tightening torque result in N·m and ft·lb.
Use "Reset" to clear all fields and start a new calculation.

Use cases

Determining torque specs for structural steel connections.
Setting torque wrench values for engine and machinery assembly.
Verifying fastener loads in bridge and civil construction.
Calculating flange bolt torques for pipe and pressure-vessel work.
Quick cross-check of manufacturer torque tables against field conditions.
Teaching engineering students the nut-factor method.

Frequently Asked Questions

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Last updated: 2026-05-23 · Reviewed by Nham Vu