How to use this calculator

Pick a bolt size to fill in the diameter and tensile stress area, then a grade to fill in the proof strength. Choose whether the joint will be taken apart again, and the condition of the threads, which sets the nut factor K. The calculator returns the preload to aim for and the torque that typically produces it.

Formulas

Proof load and recommended preloadF_p = A_t S_p F_i = 0.75 F_p (reusable) or 0.90 F_p (permanent)

Tightening torqueT = K F_i d

Tensile stress area (metric coarse, pitch p)A_t = π4 (d − 0.9382 p)²

These preload targets and nut factors follow common machine design practice, as in Shigley’s Mechanical Engineering Design.

Thread condition	Nut factor K
Non-plated, black finish	0.30
Zinc-plated	0.20
Lubricated	0.18
Cadmium-plated	0.16
Anti-seize compound	0.12
Not stated (common default)	0.20

Worked example

M12 × 1.75 class 8.8 bolt, reusable joint, K = 0.20 (the default)

Proof load: F_p = 84.3 mm² × 580 MPa = 48.9 kN.
Preload: F_i = 0.75 × 48.9 = 36.7 kN.
Torque: T = 0.20 × 36,670 N × 0.012 m = 88.0 N·m.

In US units, a 1/2-13 grade 5 bolt (A_t = 0.1419 in², S_p = 85 ksi) gives F_i = 9,046 lbf and T = 0.20 × 9,046 × 0.5 = 905 lbf·in, or 75.4 lbf·ft.

Why preload matters

A properly preloaded bolt clamps the joint so tightly that most of the external load just reduces the clamping force instead of adding to the bolt tension. That protects the bolt from fatigue and keeps the joint from separating or loosening under vibration. Too little preload is a more common cause of bolt failure than too much.

Assumptions and limits

Torque is an indirect measure of preload. Friction varies with plating, lubrication and surface condition, so actual preload commonly scatters ±25% or more for the same torque.
Proof strengths are minimum values from ISO 898-1 and SAE J429 for the size ranges shown.
Stress areas are for coarse threads (ISO metric coarse and UNC).
Gaskets, soft joint materials and critical applications need the manufacturer’s torque values or direct preload measurement.

Frequently asked questions

Should I use the lubricated or dry value?

Use the condition the bolt will actually be in when tightened. Lubricating a bolt and then applying a dry torque value can overstretch it, because more of the torque goes into tension.

Why use the tensile stress area instead of the full diameter?

Threads remove material. The tensile stress area is the effective area that carries load, based on test data; it is about 75% of the full circle for common sizes.

What is the difference between reusable and permanent?

Reusable joints are taken apart for maintenance, so preload is kept at 75% of proof load to avoid yielding the threads. Permanent joints can go to 90% for more clamp force.

For education and preliminary checks only. Follow the fastener manufacturer’s or equipment maker’s torque specification when one exists.