M8 bolt torque

An M8 x 1.25 bolt takes 25.5 N·m in class 8.8, 36.5 in 10.9 and 42.6 in 12.9, dry, at 75% of proof load (19, 27 and 31 ft-lb). Fine pitches take a little more. Oiled threads take a quarter less.

Sources checked

Torque at 75% of proof load.

Units
ThreadClassDryK 0.20Zinc platedK 0.20OiledK 0.15WaxedK 0.10Clamp load
M8 x 1.25coarseClass 4.67.39.97.39.95.57.43.64.91,388 lbf6.2 kN
Class 8.818.825.518.825.514.119.19.412.73,579 lbf15.9 kN
Class 10.926.936.526.936.520.227.313.418.25,122 lbf22.8 kN
Class 12.931.442.631.442.623.632.015.721.35,986 lbf26.6 kN
M8 x 1fineClass 4.67.810.67.810.65.97.93.95.31,487 lbf6.6 kN
Class 8.820.127.320.127.315.120.510.113.63,833 lbf17.1 kN
Class 10.928.839.028.839.021.629.314.419.55,486 lbf24.4 kN
Class 12.933.745.633.745.625.234.216.822.86,411 lbf28.5 kN

Work out M8 torque

Change grade, threads or preload

Bolt standard

As received, nothing added. K 0.20 (Portland Bolt, Fastenal).

% of proof load

The published charts use 75%.

M8 x 1.25 Class 8.8, dry, plain steel

18.8ft-lb25.5N·m

225 in-lb

0102030400204060ft-lbN·m
Clamp load
3,579 lbf (15.9 kN)
Proof load
21.2 kN

The working

  1. Tensile stress area. A = π/4 × (d − 0.9382 × P)² = π/4 × (8 − 0.9382 × 1.25)² = 36.6 mm²
  2. Proof load. Class 8.8 proof stress is 580 MPa, so A × stress = 21,228 N.
  3. Clamp load. 75% of proof load = 15,921 N.
  4. Torque. T = K × D × F = 0.2 × 8 mm × 15.92 kN = 25.5 N·m (18.8 ft-lb).

Where the M8 x 1.25 numbers come from

The M8 x 1.25 coarse thread has a tensile stress area of 36.6 mm². A Class 10.9 bolt’s proof stress is 830 MPa, so its proof load is 30,378 N, and 75% of that is a clamp load of 22,784 N.

Torque = K × D × F = 0.20 × 8 mm × 22.78 kN = 36.5 N·m (26.9 ft-lb) dry. With oil (K 0.15) it’s 27.3 N·m (20.2 ft-lb).

Other sizes

All sizes on one page: the metric bolt torque chart.

Questions

What torque for an M8 x 1.25 Class 10.9 bolt?

36.5 N·m (26.9 ft-lb) dry and 27.3 N·m (20.2 ft-lb) oiled, at 75% of proof load. That clamps the joint with 5,122 lbf (22.8 kN).

What torque for an M8 x 1.25 Class 8.8 bolt?

25.5 N·m (18.8 ft-lb) dry, at 75% of proof load; a quarter less with oiled threads.

Coarse or fine thread?

Use whichever the bolt has. The fine M8 x 1 thread has a bigger stress area than the coarse one, so it takes slightly more torque for the same clamp. A thread gauge settles it.

Does this apply to wheel nuts or engine bolts?

No. Use the vehicle maker’s value for those: see lug nut torque by vehicle. These numbers are for general-purpose steel bolts where no spec exists.

Sources

  1. Fastenal: Mechanical properties per ISO 898-1 (externally threaded fasteners): proof stress by property class (checked October 14, 2026)
  2. Engineering Toolbox: Metric bolts: minimum ultimate tensile and proof loads, with ISO 898-1 stress areas (checked October 14, 2026)
  3. Engineering Toolbox: ISO 724 metric threads: coarse and fine pitches with tensile stress areas (checked October 14, 2026)
  4. Engineering Toolbox: Threaded bolts: tensile stress area formulas for unified and ISO metric threads (checked October 14, 2026)
  5. Fastenal: Torque–tension relationship for metric fasteners, property classes 4.6, 8.8, 10.9 and 12.9 (checked October 14, 2026)
  6. Portland Bolt: Bolt torque chart: T = KDP at 75% of proof load, with K for plain, waxed and galvanized bolts (checked October 14, 2026)

Spotted a number that doesn’t match your manual or a source? Tell us; corrections are logged on the sources page.