Bolt Preload & Torque Calculator

Inputs

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Tightening torque applied to the bolt or nut.
N·m
≈0.20 dry, 0.15 lubricated, 0.10 anti-seize.
Nominal (major) thread diameter — e.g. 10 mm for an M10.
mm
Thread pitch used to estimate tensile stress area. For M10 coarse, P = 1.5 mm.
mm
Fastener proof strength used for proof load. ISO class 8.8 is commonly screened at about 600 MPa.
MPa
Target preload as a percentage of estimated proof load. 75% is a common reusable-joint starting point.
%
Approximate delivered-preload spread from torque control. 25-35% is a common screening range.
%

Results

Default result
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Preload (clamp force)(F)
15,000N

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.

Also computed

Preload low estimate(F_low)10,500N

10.5 kN · 2,360 lbf

30% below the calculated torque-control preload.

Preload high estimate(F_high)19,500N

19.5 kN · 4,384 lbf

30% above the calculated torque-control preload.

Proof-load utilization(F / Fp)Pass43.11%

Compared with 600 MPa proof strength and At from d and pitch.

Torque for target preload(T_target)52.19N·m

Torque for 75% of estimated proof load using the same K.

Target preload(F_target)26,100N

26.1 kN · 5,866 lbf

Estimated proof load(Fp)34,790N

34.79 kN · 7,822 lbf

Method notes 4 notes
  • 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.
  • Tensile stress area is estimated as At = π/4 · (d − 0.9382P)^2 for a 60-degree thread profile; use the standard stress area for the exact fastener when it matters.
  • Torque-control scatter band uses the entered ±30% around the calculated preload. K varies widely with finish, plating and lubricant — measure it if the joint matters.
  • Target torque uses 75% of estimated proof load. A common target preload is 75–90% of bolt proof load, checked against the actual fastener grade.

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