How tightening torque is calculated
A bolt works by stretching like a stiff spring and clamping the joint together. The torque you apply only matters because it creates that clamp force, the preload. The short-form nut factor equation links the two:
T = K · F · d
F = preload % × proof strength × As
- T
- tightening torque
- K
- nut factor: about 0.20 dry, 0.15 oiled, 0.12 with anti-seize
- F
- target preload, usually 75% of the proof load
- d
- nominal bolt diameter
- As
- tensile stress area of the thread
Torque values, dry threads (K = 0.20), 75% of proof load
| Bolt | Class 8.8 | Class 10.9 | Class 12.9 |
|---|---|---|---|
| M8 | 25 Nm | 36 Nm | 43 Nm |
| M10 | 50 Nm | 72 Nm | 84 Nm |
| M12 | 88 Nm | 126 Nm | 147 Nm |
| M16 | 219 Nm | 313 Nm | 365 Nm |
| M20 | 441 Nm | 610 Nm | 713 Nm |
| Bolt | SAE grade 5 | SAE grade 8 |
|---|---|---|
| 1/4-20 UNC | 8 ft-lb | 12 ft-lb |
| 3/8-16 UNC | 31 ft-lb | 44 ft-lb |
| 1/2-13 UNC | 75 ft-lb | 106 ft-lb |
| 5/8-11 UNC | 150 ft-lb | 212 ft-lb |
| 3/4-10 UNC | 266 ft-lb | 376 ft-lb |
Proof strengths from ISO 898-1 and SAE J429. Published charts differ by up to 20% because they assume different friction values.
Dry or lubricated?
Most of the torque is spent on friction, not on stretching the bolt. If a chart gives a dry value and the bolt is oiled, the same torque can overload it. Always match the torque to the actual thread condition, and follow the equipment maker's value where one is given. For cylinder forces in the same joint, see the hydraulic cylinder force calculator.