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Bolt Torque Calculator

Reference: T = K·D·F
🔩 MECH
Free bolt torque calculator. Find the tightening torque (lb-ft, lb-in, N·m) from bolt diameter, target preload and nut factor K. US units with lubricated and dry conditions, worked examples.
Inputs
Enter the bolt nominal diameter and the target preload (clamp force), and select the friction condition (nut factor K). The calculator returns the tightening torque in lb-ft, lb-in and N·m.
Results
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Engineering Intelligence

ℹ About This Calculator

The torque applied to a bolt determines the clamping (preload) force it develops, and getting it right is critical — too little and the joint loosens or leaks, too much and the bolt yields or the parts crush. This calculator computes the tightening torque required to reach a target preload using the standard short-form relationship T = K·D·F, in US units (lb-ft and lb-in) plus N·m. It accounts for the nut factor K, which captures the friction between the threads and under the bolt head for dry, lubricated or coated conditions.

Most of the torque applied to a bolt is consumed by friction — roughly half under the bolt head and half in the threads — with only a small fraction actually stretching the bolt to create preload. The nut factor K lumps all these friction effects into a single empirical coefficient, so the same target preload needs much less torque when the threads are lubricated (K≈0.15) than when dry (K≈0.20). This is why lubrication condition dramatically affects the required torque, and why torque specs must state the assumed condition. Target preload is typically 60-75% of the bolt's proof load.

Bolt Torque Formula

T = K·D·F

T = K · D · F, where T = tightening torque, K = nut factor (≈0.20 dry, 0.15 lubricated, 0.10 waxed/coated), D = nominal bolt diameter, F = target preload / clamp force. Torque in lb-in = K·D(in)·F(lb); divide by 12 for lb-ft.

Worked Example

A 1/2-inch bolt is to be tightened to a 10,000 lb preload, dry (K = 0.20). Torque T = 0.20 × 0.5 × 10,000 = 1,000 lb-in = 83.3 lb-ft (about 113 N·m). Lubricating the bolt (K = 0.15) would reduce the required torque to 62.5 lb-ft for the same preload.

Frequently Asked Questions

How do I calculate bolt tightening torque? +
Use the short-form equation T = K·D·F, where K is the nut factor (about 0.20 dry, 0.15 lubricated), D is the nominal bolt diameter, and F is the target preload or clamp force. For example, a 1/2-inch bolt at 10,000 lb preload, dry, needs 0.20 × 0.5 × 10,000 = 1,000 lb-in = 83 lb-ft. The torque produces preload mainly by overcoming friction, so the friction condition strongly affects the result.
What is the nut factor K? +
The nut factor (or torque coefficient) K is an empirical value that lumps together all the friction in a bolted joint — under the bolt head and in the threads — into one coefficient in the T = K·D·F equation. Typical values are about 0.20 for dry as-received bolts, 0.15 for lubricated, 0.10 for waxed or MoS₂-coated, and up to 0.25 for zinc-plated dry. Because most of the torque fights friction, K has a large effect on the torque-preload relationship.
Why does lubrication change the torque needed? +
Because roughly 85-90% of the torque applied to a bolt is consumed by friction, and only 10-15% actually stretches the bolt to create preload. Lubrication reduces the friction, lowering the nut factor K, so the same preload is reached with less torque. This means a torque spec developed for dry bolts will grossly over-tighten lubricated bolts — potentially yielding them — which is why the lubrication condition must always be specified with the torque.
What preload should a bolt be tightened to? +
Preload is typically targeted at 60-75% of the bolt's proof load (the maximum stress it can carry without permanent deformation), which for structural and machine bolts provides a tight, fatigue-resistant joint with a margin against yielding. High preload keeps the joint clamped so external loads are carried by the clamped members rather than cycling the bolt. The exact target depends on the application, the bolt grade and whether the joint is critical.
How accurate is torque as a way to set preload? +
Torque control is convenient but imprecise — because the nut factor varies with surface finish, lubrication, plating and reuse, the actual preload from a given torque can scatter by ±25-35%. For critical joints, more accurate methods are used: turn-of-nut, bolt-tension indicators (DTI washers), or measuring bolt elongation directly. This calculator gives the target torque from an assumed K, but real joints benefit from verifying the friction condition or using a direct-tension method.
What is the difference between lb-ft and lb-in torque? +
They are both units of torque differing by a factor of 12: 1 lb-ft = 12 lb-in. Torque wrenches for large bolts read in lb-ft (foot-pounds), while small-fastener specs are often in lb-in (inch-pounds). The bolt torque formula T = K·D·F with D in inches and F in pounds gives the result directly in lb-in; divide by 12 to get lb-ft. This calculator shows both, plus N·m for metric tools.
Is this bolt torque calculator accurate? +
It applies the standard T = K·D·F short-form equation, which is the industry-accepted method for estimating tightening torque, so it is accurate to the degree that the nut factor K matches your actual joint condition. Because K varies with lubrication, plating and surface finish, select the K that matches your fasteners, and for critical joints verify preload with a direct-tension method rather than relying on torque alone.

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