Proof load is the tensile force a bolt must carry without any permanent stretch, and it is the number to use when you are checking a bolt against a load. This chart gives the proof load and the minimum breaking load in kilonewtons for every metric coarse thread size from M3 to M39, in the four property classes you will find on a drawing or a bolt head: 4.6, 8.8, 10.9 and 12.9. An M12 class 8.8 bolt has a proof load of 48.9 kN, about 5.0 tonnes-force, and an M10 class 8.8 bolt has 33.6 kN.
Bolt proof load chart (kN)
Proof load = stress area x proof stress. The stress area is the effective cross-section of the thread, which is smaller than the plain shank, so it is the figure that governs. Class 8.8 uses a proof stress of 580 MPa up to M16 and 600 MPa above it, as ISO 898-1 sets.
| Thread size |
Stress area (mm2) |
Class 4.6 (kN) |
Class 8.8 (kN) |
Class 10.9 (kN) |
Class 12.9 (kN) |
| M3 | 5.03 | 1.1 | 2.9 | 4.2 | 4.9 |
| M4 | 8.78 | 2.0 | 5.1 | 7.3 | 8.5 |
| M5 | 14.2 | 3.2 | 8.2 | 11.8 | 13.8 |
| M6 | 20.1 | 4.5 | 11.7 | 16.7 | 19.5 |
| M8 | 36.6 | 8.2 | 21.2 | 30.4 | 35.5 |
| M10 | 58 | 13.1 | 33.6 | 48.1 | 56.3 |
| M12 | 84.3 | 19.0 | 48.9 | 70.0 | 81.8 |
| M14 | 115 | 25.9 | 66.7 | 95.5 | 111.5 |
| M16 | 157 | 35.3 | 91.1 | 130.3 | 152.3 |
| M18 | 192 | 43.2 | 115.2 | 159.4 | 186.2 |
| M20 | 245 | 55.1 | 147.0 | 203.3 | 237.7 |
| M22 | 303 | 68.2 | 181.8 | 251.5 | 293.9 |
| M24 | 353 | 79.4 | 211.8 | 293.0 | 342.4 |
| M27 | 459 | 103.3 | 275.4 | 381.0 | 445.2 |
| M30 | 561 | 126.2 | 336.6 | 465.6 | 544.2 |
| M33 | 694 | 156.2 | 416.4 | 576.0 | 673.2 |
| M36 | 817 | 183.8 | 490.2 | 678.1 | 792.5 |
| M39 | 976 | 219.6 | 585.6 | 810.1 | 946.7 |
Minimum breaking load chart (kN)
Breaking load = stress area x minimum tensile strength. This is the load at which a bolt is allowed to fail, not a load to design to. The proof load in the table above is the more useful number.
| Thread size |
Class 4.6 (kN) |
Class 8.8 (kN) |
Class 10.9 (kN) |
Class 12.9 (kN) |
| M3 | 2.0 | 4.0 | 5.2 | 6.1 |
| M4 | 3.5 | 7.0 | 9.1 | 10.7 |
| M5 | 5.7 | 11.4 | 14.8 | 17.3 |
| M6 | 8.0 | 16.1 | 20.9 | 24.5 |
| M8 | 14.6 | 29.3 | 38.1 | 44.7 |
| M10 | 23.2 | 46.4 | 60.3 | 70.8 |
| M12 | 33.7 | 67.4 | 87.7 | 102.8 |
| M14 | 46.0 | 92.0 | 119.6 | 140.3 |
| M16 | 62.8 | 125.6 | 163.3 | 191.5 |
| M18 | 76.8 | 159.4 | 199.7 | 234.2 |
| M20 | 98.0 | 203.3 | 254.8 | 298.9 |
| M22 | 121.2 | 251.5 | 315.1 | 369.7 |
| M24 | 141.2 | 293.0 | 367.1 | 430.7 |
| M27 | 183.6 | 381.0 | 477.4 | 560.0 |
| M30 | 224.4 | 465.6 | 583.4 | 684.4 |
| M33 | 277.6 | 576.0 | 721.8 | 846.7 |
| M36 | 326.8 | 678.1 | 849.7 | 996.7 |
| M39 | 390.4 | 810.1 | 1015.0 | 1190.7 |
Strength values used
These are the ISO 898-1 values behind both tables, taken from the standard and checked against further published tables.
| Property class |
Proof stress Sp (MPa) |
Minimum tensile strength Rm (MPa) |
| 4.6 | 225 | 400 |
| 8.8, up to M16 | 580 | 800 |
| 8.8, over M16 | 600 | 830 |
| 10.9 | 830 | 1040 |
| 12.9 | 970 | 1220 |
Worked example: M12 class 8.8
The stress area of an M12 coarse thread is 84.3 mm2 and class 8.8 has a proof stress of 580 MPa. Multiply them: 84.3 x 580 = 48894 N, which is 48.9 kN. The minimum breaking load is 84.3 x 800 = 67440 N, or 67.4 kN. To convert kilonewtons, multiply by 102.0 for kilograms-force or by 225 for pounds-force: the M12 proof load is about 4986 kgf or 10992 lbf.
Choosing between classes
At the same size, class 10.9 carries about 43 per cent more proof load than class 8.8, and class 12.9 about 17 per cent more than 10.9. An M16 bolt has a proof load of 91.1 kN in class 8.8 and 130.3 kN in class 10.9. The class name does not scale the load directly, because the proof stress rises more than the nominal tensile figure suggests. For an M20 class 10.9 bolt the proof load is 203.3 kN.
The two numbers in a class name tell you the strength. The first is the nominal tensile strength in hundreds of megapascals and the second is the yield-to-tensile ratio in tenths, so 8.8 is 800 MPa nominal with a yield of 0.8 times that. Torque values to reach a chosen fraction of these loads are on the metric bolt torque chart and the bolt torque calculator.
What these figures are not
Proof load is a test load on a standard bolt, not a safe working load. A real joint needs a safety factor, allowances for shear, fatigue, temperature and the weakest part of the assembly, and bolts are normally tightened to a fraction of proof load, not up to it. The M8 class 8.8 value of 21.2 kN, for instance, is the load the bolt is tested at, not what you should hang from it. The chart covers coarse thread only. ISO 898-1 property classes for these sizes stop at M39, so there are no rows above that, and fine pitch threads have a smaller stress area and a lower load. Stainless steel A2 and A4 bolts follow a different standard, ISO 3506, and are not covered here.
Related fastener charts
Head and nut sizes for the same threads are on the bolt head sizes chart and the spanner size chart. The term is defined in the glossary under proof load, and general material strength figures are on the metal strength chart.
References
- ISO 898-1:2013 - Mechanical properties of fasteners made of carbon steel and alloy steel, bolts, screws and studs with specified property classes, coarse thread and fine pitch thread
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