Bolt Torque Calculator

The bolt torque calculator estimates the tightening torque you need to apply to a fastener to reach a target clamping force, known as the preload. It uses the widely used short-form torque equation, where torque equals the nut factor K multiplied by the nominal bolt diameter and the desired preload. You enter three inputs: the bolt diameter in metres, the target preload in newtons, and the dimensionless nut factor K that lumps together thread and head friction. The result is the required torque in newton metres, the value you would dial into a torque wrench. Mechanical engineers, machine builders, maintenance technicians and serious DIYers use it to make sure a joint is clamped tightly enough to resist loosening and fatigue, without overstretching the bolt past its yield point. The nut factor matters more than people expect: a common rule of thumb is K of about 0.2 for plain, lightly oiled steel bolts, around 0.3 for dry or as-received fasteners, and as low as 0.1 to 0.15 where a good lubricant or anti-seize is used. A few tips help you tighten with confidence. First, base preload on roughly 70 to 75 percent of the bolt proof load rather than guessing. Second, use the actual lubrication condition when picking K, because the same torque can give very different clamp force on dry versus oiled threads. Third, where the joint is safety critical, prefer angle control or direct tension measurement, since torque alone is only an indirect estimate of true preload.

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T = K x D x F. Estimate only, not engineering design advice.