Bolt Torque Calculator
Find the torque needed to achieve a target bolt preload.
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Formulas
T = K × F × dK = nut factor (0.2 dry steel, 0.15 lubricated), F = preload, d = bolt diameter
Bolt Torque
K factors: Dry steel: 0.20. Lubricated: 0.15. Cadmium plated: 0.14. Anti-seize: 0.12. Only 10-15% of applied torque becomes actual clamping force; the rest is lost to friction.
How Bolt Torque Relates to Clamping Force
When you tighten a bolt, the torque you apply stretches the bolt and creates a clamping force (preload) that holds the joint together. The relationship is captured by the short-form torque equation:
T = K × F × dT is the tightening torque, K is the nut factor (a friction coefficient), F is the target preload, and d is the nominal bolt diameter.
The nut factor K bundles together thread friction and the friction under the bolt head. It is the single biggest source of uncertainty: lubrication, plating, and surface finish all change it, which is why the same torque can produce very different clamping forces.
Worked Examples
Target preload of 15 kN on an M10 bolt (d = 0.010 m), dry steel K = 0.20:
T = 0.20 × 15000 × 0.010 = 30 N·m
Lubricating the threads lowers K to about 0.15:
T = 0.15 × 15000 × 0.010 = 22.5 N·m
The same clamping force needs 25% less torque when lubricated. Applying the dry torque value to a lubricated bolt can over-stretch or snap it.
Where the Torque Actually Goes
Only a small fraction of applied torque becomes useful clamping force. Roughly 50% is lost to friction under the bolt head, another 35% to thread friction, and only the remaining 10–15% stretches the bolt to create preload. This is why controlling friction (through lubrication or plating) matters as much as the torque value itself.
Nut Factor (K) Reference
| Condition | Nut factor K |
|---|---|
| Dry steel (as received) | 0.20 |
| Lubricated (oil) | 0.15 |
| Cadmium plated | 0.14 |
| Anti-seize compound | 0.12 |
| Zinc plated | 0.22 |
These are typical values; for critical joints, use the K value specified by the fastener or lubricant manufacturer, or measure preload directly.
Frequently Asked Questions
Why does lubrication change the torque needed?
Lubrication reduces friction, so more of the applied torque converts to clamping force. If you lubricate but use the dry torque value, you risk over-tightening and yielding the bolt.
What preload should I target?
A common guideline is 75% of the bolt's proof load for reusable joints. The proof load depends on the bolt grade (such as 8.8 or 10.9) and diameter — consult a fastener strength table.
How accurate is torque control?
Torque wrenches typically achieve ±25% preload scatter because of friction variation. For precision joints, angle-controlled tightening or direct bolt-tension measurement is far more repeatable.
Is this formula valid for all bolts?
The T = K·F·d short form is a widely used approximation for standard threaded fasteners. For high-precision work, a long-form equation accounting for thread pitch and exact friction radii gives better results.