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.
🧮 110 Free MEP Calculators
Browse all HVAC, Electrical, Plumbing, Fire, Gas and Mechanical calculators - IS/IBC/ASHRAE compliant, free PDF export.
Browse All Calculators →
