Field calculator
Bolt torque calculator (T = K x D x F)
Torque is the practical stand-in for the clamp force a bolt actually delivers, and the short-form equation links them: torque equals the nut factor K times the bolt nominal diameter D times the target preload F. Enter the diameter in inches, the preload in pounds, and a K value to get the torque in inch-pounds and foot-pounds. The whole calculation lives and dies on K, the nut factor, which rolls the thread friction and the friction under the nut or bolt head into one number. K runs around 0.2 for plain dry steel, drops when the threads are lubricated, and climbs when they are rusty or galvanized, and because torque scales directly with it, the same preload can need very different torque depending on the condition of the fastener. Treat this as an estimate for when you genuinely know the preload and a defensible K. For any real joint, electrical lug, or structural connection, use the published torque value from the equipment manufacturer, the engineer, or the governing standard (such as RCSC for high-strength structural bolts), along with the specified tightening method like turn-of-nut, direct-tension indicators, or a calibrated wrench.
Result
Bolt torque from preload: T = K × D × F, where K is the nut factor, D is the nominal bolt diameter, and F is the target clamp (preload) force. Enter the diameter in inches, the preload in pounds, and a K value (the result is in inch-pounds, shown also in foot-pounds). K is the catch: it bundles the friction at the threads and under the nut, runs about 0.2 for plain dry steel, lower when lubricated, and higher when rusty or galvanized, and it swings the torque more than anything else. This is an estimate for when you know the preload and K. For a real joint, use the published torque specification from the manufacturer, the engineer, or the structural standard (such as RCSC for structural bolts), and the correct method (turn-of-nut, DTI, or calibrated wrench).
anvilfield.com/calculators/bolt-torque-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.
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Bolt torque FAQ
Does the NEC require a torque wrench for terminations?
Where the manufacturer gives a numeric torque value, recent NEC editions at 110.14(D) require a calibrated torque tool, or another approved means the manufacturer provides, to reach it. The 2017 edition introduced the calibrated-tool language and later editions reworded it. Confirm the wording against the adopted code edition.
Should you re-torque electrical connections?
No, not unless the manufacturer specifies it. A correctly torqued connection seats and relaxes slightly, and running a tool back to the original value over-tightens it and can damage the joint. NFPA 70B cautions against this. Find loosened connections by the witness mark and an infrared scan instead of re-torquing.
Where do you get the torque value for a termination?
From the equipment: the value on the label or wiring diagram, the value stamped on the lug, or the manufacturer's instructions. That is the value the connection was listed to under UL, so it overrides any generic chart. Use a fallback table only when no manufacturer value exists anywhere.
Why do you mark a torqued connection?
A witness mark, a brittle paint stripe across the fastener and its body, shows the connection was torqued and lets an inspector confirm a whole panel at a glance. Because the cured stripe cracks if the fastener turns, an offset mark flags a connection that has loosened since it was made. It is not code-required.
What torque do you use if the manufacturer gives no value?
When no value exists on the equipment, the lug, or the instructions, fall back to published tightening-torque tables, commonly the informative annex of UL 486A-486B, organized by conductor and screw size. Note in the record that the value came from a table, not the equipment, and confirm which annex your adopted code edition references.