M8 bolt tightening torque

A M8 bolt in Class 10.9, dry, torques to about 36.5 N·m (26.9 lb-ft) at 75% of proof load.

8 mm major dia · 1.25 mm pitch · A_t = 36.6 mm² · clean, undamaged threads

Torque calculator T = K·D·F

Grade / class

Lubrication

Preload, % of proof

Tightening torque

T = K \cdot D \cdot F

T torque · K nut factor (friction) · D nominal diameter · F preload

Preload (target clamp load)

F = u \cdot S_p \cdot A_t

u utilisation (0.75) · S_p proof strength · A_t tensile stress area

M8 torque by grade and lubrication (N·m)

Grade	Dry / as-received steel K=0.2	Zinc plated K=0.22	Lubricated (light oil) K=0.15	Molybdenum disulfide K=0.1	Anti-seize compound K=0.13	Waxed / PTFE K=0.1
Class 8.8	25.5	28	19.1	12.7	16.6	12.7
Class 10.9	36.5	40.1	27.3	18.2	23.7	18.2
Class 12.9	42.6	46.9	32	21.3	27.7	21.3
A2-70 Stainless	19.8	21.7	14.8	9.88	12.8	9.88

Proof strengths per ISO 898-1 (ISO 3506-1 for A2-70); tensile stress area per ISO 898-1. Values at 75 % of proof load.

Caution — match the column to your assembly. The same bolt can need half the torque lubricated versus dry, because K drops from ~0.20 to ~0.10. A dry-torque figure applied to a lubricated bolt over-tightens it and can snap it.

How the value is worked out

Torque isn't the goal — clamp load is. Tightening stretches the bolt into a controlled tension that squeezes the joint together. For a M8 in Class 12.9 at 75 % of proof load that target tension is about 26.6 kN; torque is just the indirect lever used to reach it, and the relationship runs through the nut factor K. The tensile stress area of the M8 thread is 36.6 mm²; multiply it by the grade's proof strength and by 0.75 to get the preload F, then feed F into T = K·D·F for the table above.

Choosing the right friction condition

This is the input people get wrong most often. K bundles every friction loss in the joint — under the head and across the threads — and it moves the torque dramatically: dry, a M8 in Class 12.9 wants about 42.6 N·m; with light oil only 32 N·m; with MoS₂ roughly 21.3 N·m — for the same target tension. Always match the column to what is actually on the threads.

Grade matters as much as size

Class 8.8 tolerates roughly 25.5 N·m dry; Class 12.9 in the same diameter takes about 42.6 N·m because it safely holds far more tension. Torquing a lower grade to a higher grade's table is another classic way to fail a joint — check the head markings before picking a row. And remember: torque-wrench control scatters ±25 % in practice; for safety-critical joints use angle-control or direct tension measurement.

Tapping the mating hole? M8 tap drill size · M8 clearance hole

References

ISO 898-1 — Mechanical properties of fasteners: property classes 8.8 / 10.9 / 12.9 (ISO 3506-1 for A2-70).
ISO 724 / ISO 898-1 — metric thread dimensions and tensile stress area.
Machinery’s Handbook — torque–preload relation and nut-factor scatter.

Last reviewed: 2026-07 · Calculation method & assumptions