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MachineCalcs

Bolt Preload & Torque Calculator

Clamp force (preload) from applied torque, nut factor and bolt diameter using the short-form torque–tension relation F = T/(K·d). Adjust K for dry, lubricated or anti-seize. Metric and imperial.

Inputs

N·m
mm

Results

Preload (clamp force)(F)
15 000 N

15 kN · 1.53 t · 3 372 lbf

Using K = 0.2 (≈0.20 dry, 0.15 lubricated, 0.10 anti-seize).

F = T / (K · d) — the force clamping the joint.

  • Short-form torque–tension: the nut factor K lumps thread and under-head friction together; only about 10–15% of the torque becomes preload, the rest is lost to friction.
  • K varies widely with finish, plating and lubricant — measure it if the joint matters. Torque control sets preload to only about ±25–35%.
  • A common target preload is 75–90% of the bolt proof load; check it against your fastener grade.

How it works

The short-form torque–tension relation gives the clamp force a bolt develops from the tightening torque: F = T / (K · d) where T is the applied torque, d the nominal bolt diameter, and K the nut factor. K lumps the thread friction, under-head friction and thread lead into one empirical coefficient — it is not the coefficient of friction. Most of the torque never reaches the bolt as tension: typically only about 10–15% becomes preload, and the rest is spent overcoming friction under the head and in the threads. That is why K — and anything that changes friction, like plating or lubricant — dominates the result.

Worked example

Tightening an M10 bolt (d = 10 mm) to 30 N·m dry, with K = 0.20: F = 30 / (0.20 × 0.010) ≈ 15,000 N — about a 15 kN clamp force. Lubricate the same bolt so K drops to 0.15 and the identical 30 N·m now gives F = 30 / (0.15 × 0.010) ≈ 20,000 N, about 20 kN. Same torque, a third more preload — which is exactly why lubricated bolts get a lower torque spec. The calculator returns these directly.

Frequently asked questions

How do you calculate bolt preload from torque?
Preload F = T / (K · d), where T is the applied torque, K the nut factor, and d the nominal bolt diameter. For 30 N·m on an M10 (d = 0.010 m) with K = 0.20: F = 30 / (0.20 × 0.010) ≈ 15,000 N (15 kN).
What is the nut factor (K-factor)?
K is a single empirical coefficient that lumps the thread friction, under-head friction and thread geometry into one number, so torque and preload relate as T = K · F · d. It is roughly 0.20 dry/as-received, 0.15 lubricated, and 0.10 with anti-seize.
Why does lubrication change the torque?
Lubricant lowers the friction, which lowers K. Because F = T/(K·d), a lower K means the same torque produces more preload — so lubricated bolts must be tightened to a lower torque to avoid over-stretching. Going from K = 0.20 to 0.15 raises preload by about a third for the same torque.
What torque for an M10 bolt?
It depends on the target preload and the nut factor, not just the size. As a worked figure, 30 N·m on an M10 with K = 0.20 (dry) gives about 15 kN of clamp force; set the torque to hit your target preload (typically 75–90% of proof load).
Is 75–90% of proof load the right target?
Yes — preloading to about 75–90% of the bolt’s proof load is the typical target for a reusable, properly designed joint. It keeps the joint clamped and the bolt in its elastic range; back off for soft or fatigue-sensitive joints.
Does this work in metric and imperial?
Yes — enter torque in N·m or lbf·ft and diameter in mm or inches; the preload is shown in N, kN, tonne and lbf.

Method & assumptions

  • Short-form torque–tension F = T/(K·d) with a single lumped nut factor — it does not separate thread and under-head friction.
  • K varies widely with finish, plating, coating and lubricant; the 0.20 / 0.15 / 0.10 values are typical, not exact — measure K if the joint matters.
  • Torque control is only about ±25–35% accurate on the actual preload because friction scatters; use angle or stretch control where preload is critical.

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