Bolt Torque Calculator

Blog post description.

Wiratama

11/15/20251 min read

Bolt Torque – Definition

Bolt torque is the rotational force required to properly tighten a bolt so it achieves the desired clamping force (also called preload). Correct torque is essential to ensure that bolted joints remain secure, resist loosening, and prevent structural or mechanical failure.

Background Theory

When a bolt is tightened, torque is converted into tension within the bolt, creating a clamping force between the joint surfaces. The relationship between torque and preload is commonly approximated by:

T=K⋅F⋅d

Where:

  • T = tightening torque (N·m)

  • F = preload or clamping force (N)

  • d = bolt nominal diameter (m)

  • K = nut factor, representing friction in the threads and under the bolt head

The nut factor K depends heavily on lubrication, coating, thread condition, and washer type. Typical values:

  • Dry bolt: 0.20–0.25

  • Lubricated bolt: 0.12–0.18

  • Plated bolt: ~0.15

Since roughly 85–90% of applied torque is lost to friction (not tensioning the bolt), accurate torque prediction requires an appropriate K-value.

How This Calculator Works

This calculator estimates the required bolt torque by:

  1. Asking for the desired clamping force (F)

  2. Asking for the bolt diameter (d)

  3. Allowing you to enter a nut factor (K) based on lubrication or material condition

  4. Applying the standard torque-preload equation

  5. Displaying the tightening torque in N·m

This provides a quick, reliable method for estimating bolt torque during mechanical design, maintenance, or assembly planning.