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Bolt Tightening Torque & Proof Load
Reference torque, proof load and clamp force for metric bolts by property class and friction condition.
How this is calculated
Tensile stress area from ISO 898-1, proof load = area × proof strength, target preload as a percentage of proof load (default 75%), and torque T = K·d·F where K is the friction factor. These are reference estimates; real torque depends on the actual coating, lubrication and joint.
Tensile stress area
58 mm²
Proof load
48.13 kN
Target preload
36.1 kN
Under-head bearing pressure
442 MPa
Tightening torque (range)
59.2–85.2 N·m
≈ 72.2 N·m
Reference table (this class, 75% preload)
| Size | Stress area | Proof load | Torque |
|---|---|---|---|
| M6 | 20.1 mm² | 16.7 kN | 15 N·m |
| M8 | 36.6 mm² | 30.39 kN | 36.5 N·m |
| M10 | 58 mm² | 48.13 kN | 72.2 N·m |
| M12 | 84.3 mm² | 69.94 kN | 125.9 N·m |
| M16 | 156.7 mm² | 130.03 kN | 312.1 N·m |
| M20 | 244.8 mm² | 203.18 kN | 609.5 N·m |
Reference estimates — not a torque specification
- •Torque depends heavily on the real friction (coating, lubrication, surface) — the K factor is the biggest variable.
- •For critical joints, confirm with the drawing and a torque–tension study.
- •Property class comes from material + heat treatment; we harden fasteners to class, but the torque here assumes the class is met.