Calculate the required tightening torque for metric bolts from grade and diameter. Covers Grade 4.6 through Grade 12.9 bolts used in structural and mechanical engineering.
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Bolt Torque Formula
T = K × d × F Where K = μ (nut factor ≈ 0.15–0.2) d = bolt diameter (m) F = clamp force = 0.7 × Proof Load
M20, Grade 8.8, μ=0.15: Stress area = 245 mm², Proof load = 0.7×640×245 = 109,760 N T = 0.15 × 0.020 × 109,760 = 329 Nm
Typical Torque Values (Grade 8.8)
M10
~48 Nm
M12
~83 Nm
M16
~205 Nm
M20
~403 Nm
M24
~692 Nm
Frequently Asked Questions
For an M16 Grade 8.8 bolt with μ=0.15: approximately 190–220 Nm depending on lubrication conditions. Use our calculator for precise values based on your friction coefficient.
The friction coefficient (μ) significantly affects the required torque. Dry bolts typically have μ=0.15–0.20, lightly oiled bolts μ=0.10–0.15, and zinc-plated bolts μ=0.12–0.18. Always use the appropriate value for your surface condition.