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Torque-Tension Conversion Calculator

Find the wrench torque needed to reach a target bolt preload force.

How it is calculated

The tool uses the nut-factor relation T = K × (D / 1000) × F. K is the dimensionless nut factor, D is the nominal bolt diameter in millimetres (the tool divides by 1000 to get metres), F is the target preload in newtons, and T is the wrench torque in N·m, shown with two decimals. All three inputs must be greater than zero; a zero or negative value returns no result. The tool does not know what K should be, so it trusts whatever you enter, and it does not check the figure against the bolt's strength. The result is a planning figure for engineering use, not a substitute for the joint's specification.

Worked example

A 12 mm bolt must reach 40 000 N of preload, with a nut factor of 0.2. Step 1: convert the diameter, 12 mm = 0.012 m. Step 2: multiply K and D, 0.2 × 0.012 = 0.0024. Step 3: multiply by the preload, 0.0024 × 40000 = 96. The tool prints 96.00 N·m as the torque to set on the wrench.

Try this example →

Common mistakes

  • Entering the diameter in metres or inches. The field expects millimetres, so 0.012 would give a torque 1000 times too small.
  • Entering the preload in kilonewtons or kilograms-force instead of newtons.
  • Using the same nut factor for dry and lubricated threads. Lubrication changes K a lot, and the torque scales directly with it.
  • Treating the result as exact. Friction varies bolt to bolt, so the real preload can differ noticeably from the target for the same torque.

About this calculator

The nut factor (K) method converts a target bolt preload into the wrench torque needed to achieve it, using the relationship T = K × D × F, where T is applied torque, D is the bolt’s nominal diameter, F is the desired clamp force (preload), and K is an empirical nut factor that captures friction under the bolt head, in the threads, and thread geometry — typically around 0.15 to 0.20 for clean, lubricated steel fasteners, but ranging much higher for dry or galled threads.

Mechanical and aerospace engineers, along with assembly technicians on production lines, rely on this conversion because bolted joints depend on preload — not torque itself — to stay clamped together, resist vibration loosening, and provide the fatigue resistance the joint was designed for; torque is simply the easiest quantity to measure and control on the shop floor with a torque wrench.

This calculator takes the bolt diameter, the target preload force, and the nut factor for your fastener condition and returns the tightening torque you need to dial in on the wrench.

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