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Bolt Preload Calculator

Work out bolt preload instantly with clear inputs, formula shown and shareable results.

About Bolt Preload Calculator

Preload is the tensile stress area multiplied by the target stress. Tensile area uses the unified thread relation pi/4 x (D - 0.9743/n)², which sits between the pitch and minor diameters. Structural practice targets 65-75 percent of proof strength so the bolt stays elastic while clamping hard enough to keep the joint from slipping.

Formulas

Tensile stress area

At = pi/4 x (D - 0.9743 / n)²

D
Nominal diameter
n
Threads per inch

Preload

F = At x yield strength x target fraction

Worked example

With the default values below, Bolt Preload Calculator returns Target preload of 12,771. Change any input above and every figure here is recalculated in your browser.

Inputs used

Bolt nominal diameterminimum 0.05 in
0.5 in
Threads per inchminimum 4
13
Proof or yield strengthminimum 10 ksi
120 ksi
Target percentage of yieldaccepts 10–100 %
75 %

Results

Target preload
12,771 lbf
Tensile stress area
0.1419 in²
Resulting bolt stress
90.0 ksi
Torque at K = 0.20
106.4 lb·ft

Step by step

  1. 1Values used
  2. 2Tensile stress area
  3. 3Preload
  4. 4Target preload= 12,771 lbf
  5. 5Tensile stress area= 0.1419 in²
  6. 6Resulting bolt stress= 90.0 ksi
  7. 7Torque at K = 0.20= 106.4 lb·ft

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Preliminary estimate only. Bolted joint design, including preload targets and tightening method, must be verified by a qualified engineer against the governing design code.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Bolted joint design, including preload targets and tightening method, must be verified by a qualified engineer against the governing design code.

Frequently Asked Questions

How is the Bolt Preload figure worked out?

At = pi/4 x (D - 0.9743 / n)². Preload is the tensile stress area multiplied by the target stress.

Why aim so close to yield?

A highly preloaded bolt sees very little of the external load as stress variation, so its fatigue life is far better than a loosely tightened one. The clamp also prevents joint slip and thread loosening.

What values do I need to enter?

This calculator takes 4 inputs: Bolt nominal diameter, Threads per inch, Proof or yield strength, Target percentage of yield. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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