Bolt Torque Calculator NZ

This calculator works out the preload tension, or clamping force, produced in a bolt by a given tightening torque, using the standard torque-tension relationship. When you tighten a bolt, the torque you apply stretches it slightly, and that stretch creates the tension that clamps the joint together. Getting this preload right is critical: too little and the joint can loosen or fatigue, too much and the bolt can yield or strip. The relationship between the torque applied and the tension produced is captured by a simple formula involving the bolt diameter and a nut factor, often called K, that accounts for friction in the threads and under the head. This tool computes it. You enter the tightening torque, the bolt diameter, and the nut factor, and the calculator returns the preload tension in newtons, the value in kilonewtons, the nut factor used, and the diameter for reference. The results update as you type. Use it for mechanical engineering, for setting or checking bolt tightening, or to understand why torque specifications matter. The preload tension is the torque divided by the product of the nut factor and the bolt diameter. The nut factor is the key uncertainty: a typical value for plain, as-received steel bolts is about 0.2, but it falls for lubricated threads, which produce more tension for the same torque, and rises for dry or corroded ones. This is why lubrication dramatically affects the clamping force a given torque achieves, and why torque specifications assume a particular friction condition. Because so much of the applied torque is lost to friction, only a small fraction actually goes into stretching the bolt, which is why the torque-tension relationship, though simple, must be used with a realistic nut factor for the joint's condition.

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Tightening torque (N·m)

Bolt diameter (m)

Nut factor (K)

20,833 N

preload tension

In kilonewtons20.83 kN

Nut factor0.2

Diameter12 mm

Torque-tension: tension = torque / (nut factor x diameter). Nut factor ~0.2 for plain steel, lower if lubricated. Diameter in metres (M12 = 0.012 m).

How it works

The torque-tension relationship gives the preload tension as the applied torque divided by the product of the nut factor and the bolt diameter. The nut factor bundles together the friction in the threads and under the bolt head, so a lower factor, as with lubrication, yields more tension for the same torque.

Worked example

Tightening an M12 bolt, 0.012 metres in diameter, to 50 newton metres with a nut factor of 0.2, the preload tension is 50 divided by 0.2 times 0.012, which is 50 over 0.0024, about 20,833 newtons, or 20.83 kilonewtons. Lubricating the threads to lower the nut factor would increase this tension.

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Frequently asked questions

How does torque create bolt tension?

Tightening torque stretches the bolt slightly, and that stretch produces the tension that clamps the joint. The torque-tension relationship gives the resulting preload as the torque divided by the nut factor times the bolt diameter, accounting for friction losses.

What is the nut factor K?

The nut factor bundles together the friction in the threads and under the bolt head into a single number. About 0.2 is typical for plain steel bolts as received. Lubrication lowers it, producing more tension per unit torque, while dry or corroded threads raise it.

Why does lubrication matter so much?

Most of the applied torque is consumed overcoming friction, with only a small fraction stretching the bolt. Lubricating the threads cuts that friction, lowering the nut factor, so the same torque produces noticeably more clamping tension. This is why torque specs assume a particular friction condition.