Bolt Torque Calculator
Calculate tightening torque from a target clamp load, or back-calculate clamp load from an applied torque, for a metric bolt using the torque-tension formula.
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How this calculator works
The most widely used approximation for bolt tightening torque is the torque-tension (short-form) formula:
T is tightening torque, D is nominal bolt diameter, F is the resulting clamp (preload) force, and K is the nut factor — a lumped constant that accounts for thread friction, bearing-surface friction, and thread geometry. The calculator solves this equation for whichever value you're missing.
Typical nut factor (K) values
| Condition | Typical K |
|---|---|
| Non-plated, non-lubricated steel | 0.20 – 0.22 |
| Zinc-plated steel | 0.17 – 0.20 |
| Lubricated / waxed | 0.13 – 0.16 |
| PTFE-coated | 0.10 – 0.13 |
Friction varies with surface finish, plating, and lubrication, so treat K = 0.20 as a starting estimate — use the fastener manufacturer's tested value for anything load-critical.
Frequently asked questions
Is this the same as a torque spec chart?
Published torque charts are usually built from this same formula for a specific bolt grade and K value. This calculator lets you check torque or clamp load for your own K, diameter, and target load.
Does this check if the bolt will yield?
No — this only relates torque to clamp load. Separately confirm the resulting clamp load stays below the bolt's proof load for its grade and diameter.