Bolt Preload Calculator
Estimate the clamp load (preload) generated by a given tightening torque, using the standard nut-factor method — with an optional check against the bolt's proof load.
Enter your values
Tightening torque, bolt diameter, and nut factor.
How this calculator works
The nut-factor (or "K-factor") method relates tightening torque to the resulting axial clamp load in a bolted joint:
T is the applied tightening torque, d is the nominal bolt diameter, and K is an empirical nut factor that bundles thread friction, bearing-surface friction, and thread geometry into a single coefficient. If you enter the bolt's proof load, the calculator also reports the preload as a percentage of proof load — a common target is 65–90% of proof load for a permanent structural joint.
Typical nut factor (K) values
| Condition | Typical K |
|---|---|
| As-received, non-lubricated steel | ~0.20 |
| Lubricated or cadmium/zinc plated | ~0.15 |
| PTFE-coated or waxed | ~0.10 – 0.13 |
| Dry, rough, or galled threads | 0.25+ |
K varies significantly with surface finish, lubrication, and thread condition — for critical joints, verify K experimentally or use the fastener manufacturer's published torque tables rather than relying on generic values.
Frequently asked questions
How accurate is the torque-to-preload relationship?
The nut-factor method is an estimate — friction variation alone can cause scatter of roughly ±25–30% around the calculated preload, even with consistent torque control. Where preload accuracy matters, consider load-indicating washers, angle-of-turn methods, or direct tension measurement.
How is this different from the Bolt Torque Calculator?
The Bolt Torque Calculator works from bolt grade and size to a recommended tightening torque; this calculator works the other way — from a torque you already have, back to the resulting preload.