Bolt Torque & Thread Engagement Tool

Find a tightening torque for metric and inch bolts and how much thread engagement the joint needs.

How it works

Tightening a bolt stretches it like a spring. The stretch is the clamp load (preload) that holds the joint together. Most of the torque you apply is lost to friction, so the usual short-cut relation links torque to preload through a nut factor K:

Tensile stress area (metric): As = π/4 × (d − 0.9382 P)²
Tensile stress area (inch): At = π/4 × (d − 0.9743 ÷ n)²
Proof load: Fp = Sp × As
Preload: Fi = (percent) × Fp
Torque: T = K × d × Fi

Here d is the nominal diameter, P the pitch, n the threads per inch, and Sp the proof strength of the grade. The nut factor K lumps together thread and under-head friction: about 0.20 for dry steel, lower when lubricated.

Thread engagement is estimated by comparing the bolt’s breaking load, As × Su, with the shear strength of the internal threads: about 0.6 of the tensile strength of the nut material, acting over 0.75 × π × d per unit length of engagement. A longer engagement than this means the bolt breaks before the threads strip, which is the safe way to fail. A weaker material needs a proportionally longer engagement. The recommended value shown is the larger of this calculation and the usual rule of thumb (1 × d for steel and brass, 1.5 × d for cast iron and strong aluminum, 2 × d for cast aluminum), because the calculation ignores thread tolerances, pitch errors and partial first threads.

Worked example

An M10 × 1.5 class 8.8 bolt has a stress area of 58.0 mm² and a proof strength of 580 MPa, so the proof load is 33.6 kN. At 75% preload the clamp load is 25.2 kN. With a dry thread (K = 0.20), T = 0.20 × 0.010 m × 25 230 N = 50.5 N·m (37.2 ft-lbf). Screwed into 6061-T6 aluminum (310 MPa), the strength calculation gives about 10.6 mm, roughly one bolt diameter, and the rule of thumb for aluminum is 1.5 × d, so the tool recommends 15 mm of engagement. For a 1/2-13 Grade 5 bolt at 75% preload with K = 0.20, the same steps give about 905 in-lbf, or 75 ft-lbf.

Things to know

Torque is a rough way to get preload. Friction varies with surface finish, plating, lubrication and how fast you tighten, so the real clamp load can be 25% higher or lower than calculated. For critical joints use a torque-turn method, tension indicators or direct tension.

Lubricant changes everything. Using the dry torque on an oiled bolt over-tightens it, sometimes beyond yield. If a published torque table assumes dry threads and you add oil or anti-seize, reduce the torque. Check which condition the table assumes.

Use the same condition the value was chosen for. Never use values for a different grade or finish. Stainless steel bolts gall easily, so lubricate them and tighten slowly, and treat the yield strength as the limit.

Fine threads. A fine thread has a larger stress area, so it takes a slightly higher torque than the coarse thread of the same size.

Reuse and yield. Torque-to-yield bolts, such as many engine bolts, are one-use. Never reuse them.

Engagement is an estimate. The calculation assumes full threads, a clean hole and static load. Vibration, fatigue, blind holes, thin-wall parts and helical inserts need their own checks. In a blind hole, leave extra depth beyond the bolt tip.

Gaskets, washers and sealed joints have their own torque rules from the maker. Follow them over a general calculator.

Common questions

What does preload percentage mean?

It is how close to the bolt’s proof load you tighten it. The proof load is the highest load the bolt can carry without permanent stretch. Joints you will take apart usually use about 75%, and permanent joints about 90%.

What is the K factor?

The nut factor K collects all the friction in the thread and under the head into one number so torque and preload can be linked: T = K × d × F. Typical values are 0.20 for dry steel and 0.12 to 0.16 for lubricated threads.

Why does aluminum need more thread engagement?

The bolt is stronger than the aluminum threads. The threads must have enough length to carry the bolt’s full load, or they strip before the bolt would break. Steel into aluminum usually needs about 1.5 to 2 times the bolt diameter.

Can I use this for studs and nuts?

Yes, for torque. Use “A nut of matching strength” for the engagement estimate with a standard nut of the proper grade.

Sources

  • ISO 898-1, Mechanical properties of fasteners made of carbon steel and alloy steel: bolts, screws and studs.
  • SAE J429, Mechanical and Material Requirements for Externally Threaded Fasteners.
  • Shigley’s Mechanical Engineering Design, chapter 8 (power-screw and fastener preload, T = K d F).
  • Machinery’s Handbook, screw thread shear area and length of engagement.

Last reviewed: November 2, 2026