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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.

Bolt size (coarse pitch)Property classFriction conditionTarget preload (% of proof load)

Tensile stress area

58 mm²

Proof load

48.13 kN

Target preload

36.1 kN

Tightening torque

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.

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Bolt Tightening Torque & Proof Load

How this is calculated

Reference table (this class, 75% preload)

Reference estimates — not a torque specification

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Bolt Tightening Torque & Proof Load

How this is calculated

Reference table (this class, 75% preload)

Reference estimates — not a torque specification