Preload and tightening torque
Within proof load
As and Sp, ISO 898-1:2013 · K = 0.20, assumed · 65 % of proof load · Not hot-dip galvanized or structural · T = K·Fi·d
Show the formula
Fp = As·Sp; Fi = (target / 100)·Fp; T = K·Fi·d
- As
- Tensile stress area, as ISO 898-1:2013 defines it, computed from the ISO 68-1:2023 basic profile, mm²
- Sp
- Nominal stress under proof load for the property class, ISO 898-1:2013, at the thread's diameter, MPa
- Fp
- Proof load, N
- target
- Target preload as a percentage of proof load, %
- Fi
- Preload, N
- K
- Nut factor. Assumed, not derived; common shop practice, not standardized
- d
- Nominal diameter, mm
- T
- Tightening torque. With d in mm, K·Fi·d is in N·mm and is divided by 1000, N·m
The derivation, the full assumption set and a worked example
Calculators and visualizers
Calculators
- Preload and tightening torque
Preload and tightening torque for an ISO metric bolt, from its thread, property class, nut factor and target fraction of proof load.
Cites ISO 898-1:2013, ISO 68-1:2023, ISO 261:1998, ISO 262:2023.
Visualizers
- Thread geometry and the stress area
The ISO 68-1 basic profile of the selected metric thread, drawn to scale, with d, d2, d1, d3 and the pitch dimensioned, and the tensile stress area As shown as the circle of equal area.
Cites ISO 68-1:2023, ISO 724:2023, ISO 898-1:2013.
- Preload sensitivity to the nut factor
The preload a calculated tightening torque actually gives when the real nut factor differs from the one assumed, against proof load and minimum yield.
Cites ISO 898-1:2013.
Recently revised
| Rev | Date | Page | Description |
|---|---|---|---|
| E | 2026-09-26 | Preload and tightening torque | Added the thread-geometry visualizer below the result, verified separately |
| A | 2026-09-26 | Preload sensitivity to the nut factor | Initial issue |
| A | 2026-09-26 | Thread geometry and the stress area | Initial issue |
How values are sourced
Assumptions beside the result
A torque figure is shown with the nut factor, lubrication state and target preload it rests on. Where a value is shop practice rather than a standard, the page says so in those words.
The equation is never hidden
Every calculator carries its derivation: the formula as implemented, each symbol and its unit, and a worked example you can check by hand.
Sources cited by edition
ISO 898-1:2013, not ISO 898-1. When a referenced standard is revised, the affected pages take a new revision row rather than a silent edit.
Standards cited
Each at the edition the site uses. Links go to where the standard can be obtained, never to a copy of it.