M18 bolt torque
An M18 x 2.5 bolt takes 311 N·m in class 8.8, 430 in 10.9 and 503 in 12.9, dry, at 75% of proof load (229, 317 and 371 ft-lb). Fine pitches take a little more. Oiled threads take a quarter less.
Sources checked
| Thread | Class | DryK 0.20 | Zinc platedK 0.20 | OiledK 0.15 | WaxedK 0.10 | Clamp load |
|---|---|---|---|---|---|---|
| M18 x 2.5coarse | Class 4.6 | 86.0117 | 86.0117 | 64.587.5 | 43.058.3 | 7,284 lbf32.4 kN |
| Class 8.8 | 229311 | 229311 | 172233 | 115156 | 19,423 lbf86.4 kN | |
| Class 10.9 | 317430 | 317430 | 238323 | 159215 | 26,869 lbf119.5 kN | |
| Class 12.9 | 371503 | 371503 | 278377 | 185251 | 31,401 lbf139.7 kN | |
| M18 x 1.5fine | Class 4.6 | 96.8131 | 96.8131 | 72.698.4 | 48.465.6 | 8,194 lbf36.5 kN |
| Class 8.8 | 258350 | 258350 | 194262 | 129175 | 21,851 lbf97.2 kN | |
| Class 10.9 | 357484 | 357484 | 268363 | 179242 | 30,228 lbf134.5 kN | |
| Class 12.9 | 417566 | 417566 | 313424 | 209283 | 35,326 lbf157.1 kN |
Work out M18 torque
Change grade, threads or preload
M18 x 2.5 Class 8.8, dry, plain steel
229ft-lb311N·m
- Clamp load
- 19,423 lbf (86.4 kN)
- Proof load
- 115.2 kN
The working
- Tensile stress area. A = π/4 × (d − 0.9382 × P)² = π/4 × (18 − 0.9382 × 2.5)² = 192 mm²
- Proof load. Class 8.8 proof stress is 600 MPa, so A × stress = 115,200 N.
- Clamp load. 75% of proof load = 86,400 N.
- Torque. T = K × D × F = 0.2 × 18 mm × 86.40 kN = 311 N·m (229 ft-lb).
Where the M18 x 2.5 numbers come from
The M18 x 2.5 coarse thread has a tensile stress area of 192 mm². A Class 10.9 bolt’s proof stress is 830 MPa, so its proof load is 159,360 N, and 75% of that is a clamp load of 119,520 N.
Torque = K × D × F = 0.20 × 18 mm × 119.52 kN = 430 N·m (317 ft-lb) dry. With oil (K 0.15) it’s 323 N·m (238 ft-lb).
Other sizes
All sizes on one page: the metric bolt torque chart.
Questions
What torque for an M18 x 2.5 Class 10.9 bolt?
430 N·m (317 ft-lb) dry and 323 N·m (238 ft-lb) oiled, at 75% of proof load. That clamps the joint with 26,869 lbf (119.5 kN).
What torque for an M18 x 2.5 Class 8.8 bolt?
311 N·m (229 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 M18 x 1.5 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
- Fastenal: Mechanical properties per ISO 898-1 (externally threaded fasteners): proof stress by property class (checked October 14, 2026)
- Engineering Toolbox: Metric bolts: minimum ultimate tensile and proof loads, with ISO 898-1 stress areas (checked October 14, 2026)
- Engineering Toolbox: ISO 724 metric threads: coarse and fine pitches with tensile stress areas (checked October 14, 2026)
- Engineering Toolbox: Threaded bolts: tensile stress area formulas for unified and ISO metric threads (checked October 14, 2026)
- Fastenal: Torque–tension relationship for metric fasteners, property classes 4.6, 8.8, 10.9 and 12.9 (checked October 14, 2026)
- 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.