Bolt Torque Calculator

The nut factor lumps thread friction, underhead friction, and thread geometry into one empirical number, and torque is directly proportional to it in T = K·D·F. Lubricants or anti-seize compounds cut friction and lower K, so a lubricated bolt reaches the same clamping force at a noticeably lower applied torque than the same bolt run dry.

The tensile stress area formula At ≈ 0.7854×(d − 0.9743/n)² depends on the thread pitch n, which the calculator looks up from a built-in table of standard UNC pitches keyed to diameter. Metric bolts, UNF fine-thread bolts, or custom pitches use different thread geometry, so if your bolt isn't a standard UNC size, the tensile area — and everything derived from it, like bolt stress and % of proof load — won't be accurate.

Percent of proof load compares the stress your target clamp force creates in the bolt against the proof load stress rating, and exceeding roughly 85-90% risks permanently stretching or yielding the bolt during tightening itself, before the joint even sees service loads. Staying below that threshold with margin keeps you clamping in the bolt's elastic range where preload stays stable over time.

Torque-controlled tightening only measures the torque you apply, not the clamp force it actually produces, and friction at the threads and under the bolt head can vary substantially between bolts even at the same nominal K value — from surface condition, contamination, or wear. That's why critical joints often switch to turn-of-nut methods, torque-to-yield tightening, or direct ultrasonic preload measurement instead of relying on a torque wrench and a nominal nut factor alone.