How to use the bolt preload calculator
A bolted joint is held together by the preload (clamp load) the fastener develops, not by the torque itself. The relationship used on virtually every shop floor is T = K × d × F, where T is tightening torque, K is the nut factor, d is the nominal bolt diameter and F is the preload. Solved for preload it becomes F = T ÷ (K × d). This calculator applies that formula in inch-pound units (lb-ft, inches, lbf), then divides the preload by the UNC tensile stress area to show the tensile stress in the bolt.
The nut factor K depends almost entirely on friction at the threads and under the head. Commonly published values are 0.30 for plain black-finish steel, 0.20 for zinc plated, 0.18 for oil lubricated and 0.12 for moly-disulfide grease. Dropping K from 0.30 to 0.18 at the same torque raises preload by roughly 70%, so any change in lubrication or plating means the torque spec has to be recalculated. Torque control typically scatters preload by about ±25 to 30%, which is why critical joints often use turn-of-nut, bolt stretch or direct tension indicators instead.
The target preload shown here is 75% of the SAE J429 proof load (proof strength × tensile stress area), the usual figure for joints that will be taken apart and reused; permanent joints are often taken to 90%. Treat every number as a design-check reference and have structural or pressure-boundary connections reviewed by a licensed professional engineer before the final specification is issued.
Frequently asked questions
Typical published values are 0.30 for plain black finish, 0.20 for zinc plated, 0.18 for oil lubricated and 0.12 for moly-disulfide grease. If your assembly spec lists a K value, always use that one instead.
The bolt is being pulled into the range where permanent stretch can begin. Lower the torque, or step up to a higher grade or a larger diameter and check again.