Shop tool
Estimated tightening torque for a target preload of 75% of proof load, from T = K · F · d. The nut-factor (K) assumption is shown on every result — it is the single biggest source of torque uncertainty.
M8 × 1.25 · preload 75% of proof
| Grade / class | Preload | Torque |
|---|---|---|
| Class 8.8 | 15.92 kN | 26 N·m |
| Class 10.9 | 22.79 kN | 37 N·m |
| Class 12.9 | 26.63 kN | 43 N·m |
The short-form torque equation is T = K · F · d, where F is the target preload, d the nominal diameter and K the nut factor — a lumped friction coefficient for the threads and the bearing face. A dry, as-received steel fastener is usually taken as K = 0.20; oil drops it toward 0.18, and wax or anti-seize can push it to 0.12 or lower.
Because torque scales directly with K, a lubricated joint torqued to a dry-K figure will be badly over-preloaded. That is why ThreadRef always prints the K it used, and why the lubrication selector is part of the Premium calculator.
These are estimates for reference. Always follow the fastener manufacturer's torque specification and your joint's engineering requirements for anything load-bearing.