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About this tool
Estimate clamp load and preload from bolt diameter, applied torque, and nut factor (K).
The Bolt Torque & Preload Calculator applies the short-form torque equation T = K x F x d, converting an applied torque in newton-metres into the clamp load a fastener of a given nominal diameter should produce for a chosen nut factor K, and working the same relationship backwards to give the torque needed to hit a target preload. It is for anyone specifying or sanity-checking a tightening spec — a mechanic questioning a workshop figure, or an engineer comparing a dry and a lubricated assembly. The equation is a first-order estimate; approved fastener procedures govern real assemblies.
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In the Inputs panel enter Nominal diameter (mm), Applied torque (N·m), Nut factor K and Proof load target (kN).
Tap the M10 · 45 N·m example chip to load a worked set, or Reset to put every field back to its default — the Result recomputes on each change.
Read the "… kN estimated preload" headline with the Torque for entered target and Estimated % of entered target rows, then use Copy or Download to save bolt-torque-preload-calculator.txt.
Gives you preload from torque and the torque needed for a target preload in the same result, so you do not have to rearrange anything by hand.
K is an input rather than a hidden constant, which surfaces the fact that friction — not bolt strength — dominates the torque-to-preload relationship.
Reports the estimated preload as a percentage of the proof load you entered, so you can see immediately whether the spec is under or over.
T = K x F x d, where T is torque in newton-metres, K is the dimensionless nut factor, F is the preload in newtons and d is the nominal bolt diameter in metres. Rearranged, preload F = T / (K x d) — so 45 N·m on an M10 bolt with K = 0.2 gives about 22.5 kN of clamp load.
K = 0.2 is the conventional starting value for plain, as-received steel fasteners, with lubricated or coated threads typically falling lower and dry, rusty or galled ones running higher. K is not a material property — it lumps thread friction, under-head friction and thread geometry into one number, so it should come from test data or the fastener supplier for anything load-bearing.
Because roughly 90 percent of applied torque is consumed by friction under the head and in the threads, so lubrication, plating, surface finish, reuse and even wash-down before assembly all shift the result. Torque control alone typically scatters preload by tens of percent, which is why critical joints use angle control, bolt stretch measurement or ultrasonic methods instead.
No. This is a simplified first-order estimate that ignores joint stiffness, embedment relaxation, thermal effects, gasket creep and the fastener's actual grade and proof strength. Use the manufacturer's or standard's published tightening procedure for anything structural, pressure-retaining or safety-related, and treat this as a cross-check on the order of magnitude.
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