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Bolt Torque Calculator

Find the torque to reach a bolt's clamp load.

How it is calculated

The tool uses the short-form torque equation T = K × D × F. K is the dimensionless nut factor, D the nominal bolt diameter in millimetres (converted to metres inside the tool), and F the clamp force you want in newtons. The result T is in newton-metres, and it is also given in foot-pounds by multiplying by 0.737562. If the nut factor is left empty or is not positive, the tool uses 0.2. Diameter must be above zero and force cannot be negative. The formula folds thread and under-head friction into one number, so the result is only as good as the K you choose. It does not check the bolt's strength grade or proof load.

Worked example

A 10 mm bolt should give a clamp force of 20,000 N (20 kN) with a nut factor of 0.2. Convert the diameter: 10 mm = 0.01 m. Then T = 0.2 × 0.01 × 20,000 = 40.00 N·m. In foot-pounds that is 40 × 0.737562 = 29.50 ft·lb. A torque wrench set to 40 N·m would therefore aim for about 20 kN of preload, provided the friction really matches a K of 0.2. A lubricated bolt with K = 0.15 would reach the same force at only 30 N·m.

Try this example →

Common mistakes

  • Entering the force in kN or kgf. The field takes newtons, so 20 kN must be typed as 20000.
  • Using the dry K of 0.2 for a lubricated or plated bolt. With a lower friction, the same torque gives a much higher preload and can overload the bolt; use a K that matches the actual surface condition.
  • Entering the diameter in inches or the thread pitch instead of the nominal diameter in millimetres.
  • Treating the result as exact. Real friction varies bolt to bolt, so the actual preload from a given torque can scatter widely.
  • Setting the target force above what the bolt can carry. The tool does not check bolt grade, so confirm the clamp force against the fastener's rated load first.

About this calculator

This calculator applies the standard fastener torque formula T = K·D·F, where T is the required tightening torque, K is the nut factor (a dimensionless friction coefficient, typically around 0.2 for dry, uncoated steel but lower for lubricated or coated fasteners), D is the bolt's nominal diameter, and F is the target clamp (preload) force the joint needs to stay tight under load.

Mechanical engineers, machinists, and automotive and aerospace assemblers use torque-to-tension specifications like this to avoid two failure modes: under-torquing, which leaves a joint loose and prone to vibration loosening or fatigue, and over-torquing, which can yield or snap the bolt or crush a gasket. Enter the nut factor, diameter, and target clamp force to get the torque wrench setting.

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