Bolt Torque Calculator

Bolt torque is the rotational force applied to a bolt to achieve a desired clamping load between two mating surfaces. When you tighten a bolt, torque converts into axial tension in the bolt shank, which generates the clamping force that holds the joint together. Too little torque leads to loose joints; too much can yield or break the bolt.

The standard formula is T = K × F × d × (1 − l/100), where T is the tightening torque, K is the nut factor (torque coefficient), F is the axial clamping force, d is the nominal bolt diameter, and l is the lubrication reduction percentage. This is a widely used approximation — always verify against manufacturer specifications for critical applications.

The K-factor (also called the nut factor or torque coefficient) accounts for friction between the bolt threads and the mating surface. Typical values are: 0.20 for dry mild steel or zinc-plated bolts, 0.30 for black-finish bolts, 0.18 for slightly lubricated bolts, and 0.16 for cadmium-plated bolts. Always check the bolt manufacturer's data for precision applications.

Lubrication reduces friction between the bolt threads and nut, meaning less torque is needed to achieve the same clamping force. For example, a well-lubricated bolt joint may require 30–40% less torque than a dry one. This calculator lets you input the lubrication reduction percentage to account for this effect.

For a standard M10 steel bolt (10 mm diameter) with a typical axial force of around 25,000 N and a K-factor of 0.20 (dry), the tightening torque is approximately T = 0.20 × 25,000 × 0.010 = 50 Nm. Actual recommended torques vary by bolt grade and application — always consult a torque specification table for your bolt class (e.g., ISO 898).

ISO 68 and ISO 261 cover metric bolt thread profiles, while ISO 898 specifies mechanical properties and recommended tightening torques for steel bolts. For critical or safety-relevant fastener applications, always reference the relevant ISO or ASME standard rather than relying solely on a general formula.

This calculator uses the widely accepted general formula T = K × F × d × (1 − l/100). However, it is an approximation — it does not account for screw thread pitch, thread geometry, or bearing surface friction separately. Torque wrenches themselves typically have an accuracy of ±25%. For safety-critical applications, always verify results against manufacturer data and applicable standards.

Yes. Select 'Metric' to work in millimeters, Newtons, and Newton-metres, or 'Imperial' to work in inches, pound-force, and lbf·ft. The underlying formula is the same — only the units change. Make sure all inputs are in consistent units before calculating.