Bolt tightening torque

Tightening a bolt to a chosen clamp load is a friction-dominated conversion: the wrench torque you apply is not the axial preload that holds the joint. Designers, inspectors, and assembly technicians therefore need a repeatable map from nominal diameter, friction coefficient, and desired clamp load into a tightening torque in newton-metres. This bolt torque calculator applies the Motosh short-form relation used in VDI 2230 and ISO 16047, with typical ISO metric coarse geometry for pitch, pitch diameter, and under-head bearing diameter, and returns the required torque together with the implied nut factor.

What bolt tightening torque means and how Motosh splits friction

Bolt tightening torque is the wrench moment you must apply so that the fastener develops a chosen axial clamp load, also called preload. Unlike a free lever, a threaded fastener converts rotation into tension through the helix, while friction under the nut or bolt head and friction on the thread flanks consume most of the applied work. Empirical joint studies and the VDI 2230 guideline both show that only a modest fraction of tightening torque becomes useful clamp force; the rest fights friction.

The Motosh short-form equation used here writes that energy split as torque T equals clamp load F times the sum of a pitch term 0.16 P, a thread-friction term 0.58 μ times pitch diameter d2, and an under-head term one-half μ times mean bearing diameter Dw. Diameter d is the nominal major diameter in metres, μ is a single coefficient of friction applied to both thread and bearing surfaces, and F is the desired clamp load in newtons.

When pitch is not an input, this calculator takes P as 0.15 d, the ISO metric coarse ratio that is exact for an M10 × 1.5 bolt and representative of common M8–M12 hardware; pitch diameter follows ISO 68-1 as d minus three-root-three over eight times P; mean bearing diameter is 1.3 d, a typical hex-head value.

Search intent for bolt torque calculator, bolt tightening torque, clamp load to torque, and fastener preload torque is this conversion — not a lever-arm physics torque and not a unit conversion among newton-metres and pound-feet. Keep units consistent: metres and newtons yield newton-metres. The implied nut factor K equals T divided by F times d, so you can compare the result with handbook K values of about 0.16–0.22 for as-received steel.

Where a bolt torque calculator earns its keep, and what it is not

Use the browser tool when you want an immediate newton-metre target while writing an assembly specification, checking a torque wrench setting against a clamp-load goal, or teaching why most tightening work is friction rather than useful stretch. Results are fully deterministic: identical inputs always produce the same torque, the same nut factor, and the same geometric intermediates. There is no language model, no external fastener-data service, and no randomness, so the capacity is safe to pin in continuous-integration checks and reproducible course materials.

This page is not the lever-arm torque calculator, whose formula is tau equals r F sin theta and answers a different search. It is also not a torque unit converter among newton-metres, newton-centimetres, and pound-feet. It does not replace a full VDI 2230 joint calculation with embedding, stiffness, and tightening-factor scatter; it returns the Motosh short-form torque for a chosen clamp load and a single friction coefficient.