M20 bolt torque
An M20 x 2.5 bolt takes 441 N·m in class 8.8, 610 in 10.9 and 713 in 12.9, dry, at 75% of proof load (325, 450 and 526 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 |
|---|---|---|---|---|---|---|
| M20 x 2.5coarse | Class 4.6 | 122165 | 122165 | 91.5124 | 61.082.7 | 9,294 lbf41.3 kN |
| Class 8.8 | 325441 | 325441 | 244331 | 163221 | 24,785 lbf110.3 kN | |
| Class 10.9 | 450610 | 450610 | 337458 | 225305 | 34,286 lbf152.5 kN | |
| Class 12.9 | 526713 | 526713 | 394535 | 263356 | 40,069 lbf178.2 kN | |
| M20 x 1.5fine | Class 4.6 | 135184 | 135184 | 102138 | 67.791.8 | 10,319 lbf45.9 kN |
| Class 8.8 | 361490 | 361490 | 271367 | 181245 | 27,517 lbf122.4 kN | |
| Class 10.9 | 500677 | 500677 | 375508 | 250339 | 38,065 lbf169.3 kN | |
| Class 12.9 | 584792 | 584792 | 438594 | 292396 | 44,485 lbf197.9 kN |
Work out M20 torque
Change grade, threads or preload
M20 x 2.5 Class 8.8, dry, plain steel
325ft-lb441N·m
- Clamp load
- 24,785 lbf (110.3 kN)
- Proof load
- 147.0 kN
The working
- Tensile stress area. A = π/4 × (d − 0.9382 × P)² = π/4 × (20 − 0.9382 × 2.5)² = 245 mm²
- Proof load. Class 8.8 proof stress is 600 MPa, so A × stress = 147,000 N.
- Clamp load. 75% of proof load = 110,250 N.
- Torque. T = K × D × F = 0.2 × 20 mm × 110.25 kN = 441 N·m (325 ft-lb).
Where the M20 x 2.5 numbers come from
The M20 x 2.5 coarse thread has a tensile stress area of 245 mm². A Class 10.9 bolt’s proof stress is 830 MPa, so its proof load is 203,350 N, and 75% of that is a clamp load of 152,513 N.
Torque = K × D × F = 0.20 × 20 mm × 152.51 kN = 610 N·m (450 ft-lb) dry. With oil (K 0.15) it’s 458 N·m (337 ft-lb).
Other sizes
All sizes on one page: the metric bolt torque chart.
Questions
What torque for an M20 x 2.5 Class 10.9 bolt?
610 N·m (450 ft-lb) dry and 458 N·m (337 ft-lb) oiled, at 75% of proof load. That clamps the joint with 34,286 lbf (152.5 kN).
What torque for an M20 x 2.5 Class 8.8 bolt?
441 N·m (325 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 M20 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.