Engineering Tools

Reference data for specifying and sourcing fasteners correctly — the numbers a procurement or design engineer actually needs.

Recommended tightening torque

Dry, unlubricated, reference values in Nm — always confirm against the fastener manufacturer's datasheet before use in a critical joint.

Methodology: T = K × D × F, where F = 0.75 × A_s × S_p. K = 0.2 (dry/black-oxide nut factor), D = nominal diameter, A_s = ISO metric tensile stress area, S_p = proof stress per property class (ISO 898-1 for steel, ISO 3506-1 for stainless). Target is 75% of proof load — tightening to 100% risks yielding once torsional stress from turning combines with tensile stress.

Carbon / alloy steel (ISO 898-1)

Size	Grade 8.8	Grade 10.9	Grade 12.9
M6	10.9 Nm	15.0 Nm	17.6 Nm
M8	26.4 Nm	36.5 Nm	42.6 Nm
M10	52.2 Nm	72.2 Nm	84.4 Nm
M12	91.0 Nm	126.0 Nm	147.3 Nm
M16	226.1 Nm	312.7 Nm	365.5 Nm
M20	441.0 Nm	610.1 Nm	713.0 Nm

Mild steel (unhardened, ISO 898-1)

Size	Grade 4.6	Grade 4.8	Grade 5.6	Grade 5.8
M6	4.1 Nm	5.6 Nm	5.1 Nm	6.9 Nm
M8	9.9 Nm	13.6 Nm	12.3 Nm	16.7 Nm
M10	19.6 Nm	27.0 Nm	24.4 Nm	33.1 Nm
M12	34.1 Nm	47.0 Nm	42.5 Nm	57.7 Nm
M16	84.8 Nm	116.8 Nm	105.5 Nm	143.2 Nm
M20	165.4 Nm	227.9 Nm	205.8 Nm	279.3 Nm

Stainless steel (ISO 3506-1)

A2 (304-type) and A4 (316-type) share the same mechanical property within a strength grade — A4 differs in corrosion resistance, not strength.

Size	A2-70 / A4-70	A2-80 / A4-80
M6	8.1 Nm	10.9 Nm
M8	19.8 Nm	26.4 Nm
M10	39.2 Nm	52.2 Nm
M12	68.3 Nm	91.0 Nm
M16	169.6 Nm	226.1 Nm
M20	330.8 Nm	441.0 Nm

Stainless steel has a higher, less predictable friction coefficient than plated carbon steel and is prone to thread galling. Use anti-seize or a lubricated fastener, and consider running 10–20% below the calculated dry torque as a safety margin.