Estimated Bolt Tightening Torque (Metric)
Preload and tightening torque for metric bolts in classes 8.8, 10.9 and 12.9, dry or lubricated.
A bolt holds a joint together by preload: the tension left in it after tightening. Preload is hard to measure, so it is usually set by torque. The usual relation is T = K × F × d, where T is the tightening torque, F the preload, d the nominal diameter and K a friction factor of about 0.2 for dry steel threads and 0.15 for lightly oiled ones.
This table works that out for the common sizes and classes. Each preload is 75 percent of the bolt’s proof load, a common target for a reusable joint.
The table
| Thread | Stress area (mm2) | 8.8 preload (kN) | 8.8 torque dry (N m) | 8.8 torque lubricated (N m) | 10.9 preload (kN) | 10.9 torque dry (N m) | 10.9 torque lubricated (N m) | 12.9 preload (kN) | 12.9 torque dry (N m) | 12.9 torque lubricated (N m) |
|---|---|---|---|---|---|---|---|---|---|---|
| M5 | 14.2 | 6.2 | 6.2 | 4.6 | 8.8 | 8.8 | 6.6 | 10.3 | 10.3 | 7.7 |
| M6 | 20.1 | 8.8 | 10.5 | 7.9 | 12.5 | 15 | 11.3 | 14.6 | 17.6 | 13.2 |
| M8 | 36.6 | 15.9 | 25.5 | 19.1 | 22.8 | 36.5 | 27.3 | 26.6 | 42.6 | 32 |
| M10 | 58 | 25.2 | 50.5 | 37.8 | 36.1 | 72.2 | 54.1 | 42.2 | 84.4 | 63.3 |
| M12 | 84.3 | 36.7 | 88 | 66 | 52.5 | 125.9 | 94.4 | 61.3 | 147.1 | 110.3 |
| M14 | 115.4 | 50.2 | 140.6 | 105.5 | 71.9 | 201.2 | 150.9 | 84 | 235.1 | 176.4 |
| M16 | 156.7 | 68.2 | 218.1 | 163.6 | 97.5 | 312.1 | 234.1 | 114 | 364.7 | 273.5 |
| M20 | 244.8 | 110.2 | 440.6 | 330.5 | 152.4 | 609.5 | 457.2 | 178.1 | 712.4 | 534.3 |
| M24 | 352.5 | 158.6 | 761.4 | 571.1 | 219.4 | 1053.3 | 790 | 256.4 | 1230.9 | 923.2 |
| M30 | 560.6 | 252.3 | 1513.6 | 1135.2 | 349 | 2093.8 | 1570.3 | 407.8 | 2447 | 1835.2 |
| M36 | 816.7 | 367.5 | 2646.2 | 1984.6 | 508.4 | 3660.5 | 2745.4 | 594.2 | 4278 | 3208.5 |
Source and notes: Computed: preload F = 0.75 x proof strength x stress area (proof strength from ISO 898-1: 580 MPa for class 8.8 up to M16 and 600 MPa above, 830 MPa for 10.9, 970 MPa for 12.9); torque T = K x F x d with K = 0.20 for dry plain steel threads and K = 0.15 for lightly oiled threads. K varies from about 0.1 to 0.3 with plating, lubricant and finish, so these are estimates: use the figure on the drawing or in the maker's data where there is one.
How to use this table
- Find the thread and the class stamped on the head.
- Read the preload and the torque for the way the bolt is finished: dry (plain, clean) or lubricated (oiled or waxed).
- Tighten with a calibrated torque wrench, in two or three steps, in a cross pattern if there is more than one bolt.
- If a drawing or the manufacturer gives a torque, use that figure instead.
Worked example: an M12 class 10.9 bolt
- Preload at 75 percent of proof: 52.5 kN. Torque: 125.9 N·m dry, or 94.4 N·m lubricated.
- If you apply the dry torque to a bolt that is actually well oiled (K = 0.15), the preload becomes 125.9 ÷ (0.15 × 0.012) = 69.9 kN, which is 133 percent of the target and 100 percent of the proof load.
- That is why the finish and lubricant matter as much as the torque value.
Notes and cautions
- The scatter is large. Torque control gives a preload within about ±25 to 35 percent; torque-angle tightening or a direct tension indicator is far better for critical joints.
- Plating, lubricant, thread condition, the surface under the head and re-use all change K. Treat the table as a starting point.
- Class 8.8 uses a proof strength of 580 MPa up to M16 and 600 MPa above, as ISO 898-1 gives.
- Do not use these values for bolts in plastic, in soft aluminum or through gaskets, where the clamped material limits the load.
Related tools and guides
- Bolt Torque and Thread Engagement Tool
- Bolted and Welded Joints (guide)
- Bolt Strength Grades and Property Classes
- Metric Thread Table (Coarse)
- All reference tables
FAQ
What torque for an M10 bolt?
About 50 N·m dry (or 38 N·m lubricated) for class 8.8, 72 N·m dry for 10.9 and 84 N·m dry for 12.9, from this table. Use the figure on the drawing where there is one.
Why is the lubricated torque lower?
Less friction in the threads and under the head means less of the torque is wasted overcoming friction, so a smaller torque gives the same preload.
Should I torque a bolt to its proof load?
Normally to 60 to 75 percent of it. Tightening to the full proof load leaves no margin for the working load, and any error in the friction factor can take the bolt past yield.
Sources
- ISO 898-1 for proof strength; the torque relation T = K F d is the standard short-form equation in Shigley’s Mechanical Engineering Design and in VDI 2230.
- Computed: preload F = 0.75 x proof strength x stress area (proof strength from ISO 898-1: 580 MPa for class 8.8 up to M16 and 600 MPa above, 830 MPa for 10.9, 970 MPa for 12.9); torque T = K x F x d with K = 0.20 for dry plain steel threads and K = 0.15 for lightly oiled threads. K varies from about 0.1 to 0.3 with plating, lubricant and finish, so these are estimates: use the figure on the drawing or in the maker's data where there is one.