Bolt Strength Grades and Property Classes
ISO 898-1 property classes 4.6 to 12.9 and SAE J429 grades 2, 5 and 8, in MPa and ksi.
The strength of a bolt is stamped on its head. Metric bolts carry a property class such as 8.8 or 10.9 (ISO 898-1); inch bolts carry radial lines for an SAE grade (J429). The two numbers in a class tell you the strength: the first, multiplied by 100, is the minimum tensile strength in MPa, and the second, divided by 10, is the ratio of yield to tensile strength. So class 8.8 is 800 MPa tensile with a yield of 80 percent of that, 640 MPa.
The table lists the minimum proof, yield and tensile strength of the common classes and grades in both MPa and ksi, so metric and inch hardware can be compared directly.
The table
| Standard | Grade or class | Size range | Proof strength (MPa) | Yield strength (MPa) | Tensile strength (MPa) | Proof strength (ksi) | Yield strength (ksi) | Tensile strength (ksi) | Head marking |
|---|---|---|---|---|---|---|---|---|---|
| ISO 898-1 | 4.6 | M5 to M100 | 225 | 240 | 400 | 32.6 | 34.8 | 58 | Class number on the head |
| ISO 898-1 | 4.8 | M5 to M100 | 310 | 340 | 420 | 45 | 49.3 | 60.9 | Class number on the head |
| ISO 898-1 | 5.6 | M5 to M100 | 280 | 300 | 500 | 40.6 | 43.5 | 72.5 | Class number on the head |
| ISO 898-1 | 5.8 | M5 to M24 | 380 | 420 | 520 | 55.1 | 60.9 | 75.4 | Class number on the head |
| ISO 898-1 | 6.8 | M5 to M24 | 440 | 480 | 600 | 63.8 | 69.6 | 87 | Class number on the head |
| ISO 898-1 | 8.8 | M16 and smaller | 580 | 640 | 800 | 84.1 | 92.8 | 116 | Class number on the head |
| ISO 898-1 | 8.8 | Larger than M16 | 600 | 660 | 830 | 87 | 95.7 | 120.4 | Class number on the head |
| ISO 898-1 | 9.8 | M16 and smaller | 650 | 720 | 900 | 94.3 | 104.4 | 130.5 | Class number on the head |
| ISO 898-1 | 10.9 | M5 to M100 | 830 | 940 | 1040 | 120.4 | 136.3 | 150.8 | Class number on the head |
| ISO 898-1 | 12.9 | M5 to M100 | 970 | 1100 | 1220 | 140.7 | 159.5 | 176.9 | Class number on the head |
| ASTM A307 | Grade A | 1/4 to 4 in | – | – | 414 | – | – | 60 | None (an A307 mark on the head is optional) |
| SAE J429 | Grade 2 | 1/4 to 3/4 in | 379 | 393 | 510 | 55 | 57 | 74 | No marks |
| SAE J429 | Grade 2 | Over 3/4 to 1-1/2 in | 228 | 248 | 414 | 33 | 36 | 60 | No marks |
| SAE J429 | Grade 5 | 1/4 to 1 in | 586 | 634 | 827 | 85 | 92 | 120 | Three radial lines |
| SAE J429 | Grade 5 | Over 1 to 1-1/2 in | 510 | 558 | 724 | 74 | 81 | 105 | Three radial lines |
| SAE J429 | Grade 8 | 1/4 to 1-1/2 in | 827 | 896 | 1034 | 120 | 130 | 150 | Six radial lines |
Source and notes: ISO 898-1 (minimum values for carbon and alloy steel bolts, screws and studs; yield is the 0.2 percent proof stress, or the lower yield for classes 4.6 to 5.6), SAE J429 and ASTM A307 (minimum values in ksi). The MPa and ksi columns are converted from the standard values (1 ksi = 6.894757 MPa). Class 4.8 and 5.8 yield values are minimums for the whole size range; check the standard for the exact limits by size.
How to use this table
- Find the class or grade stamped on the bolt head, and the size range it falls in.
- Use the proof strength to find the load a bolt carries without permanent stretch: multiply it by the tensile stress area from the thread tables.
- Use the tensile strength for the load at which the bolt breaks.
- To replace one bolt with another, choose a class or grade with at least the same proof strength.
Worked example: proof load of an M10 bolt in three classes
- The M10 coarse thread has a tensile stress area of 58.0 mm².
- Class 8.8 (proof 580 MPa): 33.6 kN.
- Class 10.9 (proof 830 MPa): 48.1 kN, 43 percent more than 8.8.
- Class 12.9 (proof 970 MPa): 56.3 kN.
Notes and cautions
- SAE grade 5 (120 ksi tensile) is close to ISO class 8.8, and SAE grade 8 (150 ksi) is close to class 10.9. They are not identical, so do not treat them as interchangeable in critical joints.
- Stainless steel bolts use a different system (ISO 3506): A2-70 means austenitic stainless with a minimum tensile strength of 700 MPa and a yield of about 450 MPa; A4-80 is 800 MPa and about 600 MPa.
- Very hard bolts (class 12.9, grade 8 and above) can crack from hydrogen embrittlement if they are electroplated without a baking treatment, and they are less tolerant of shock and misalignment.
- Never reuse a bolt that has been tightened past its yield, and never replace a high-strength bolt with a plain unmarked one.
Related tools and guides
- Bolt Torque and Thread Engagement Tool
- Bolted and Welded Joints (guide)
- Estimated Bolt Tightening Torque (Metric)
- Metric Thread Table (Coarse)
- All reference tables
FAQ
What does 8.8 mean on a bolt?
A property class: minimum tensile strength 800 MPa (8 × 100), and a yield strength of 80 percent of that (the second 8 divided by 10), which is 640 MPa.
What is the difference between proof strength and yield strength?
Proof strength is the stress a bolt must carry without any permanent stretch; it is the basis for choosing the tightening load. Yield strength is the stress at which the steel starts to stretch permanently, a little above the proof strength.
Is a grade 8 bolt the same as a 10.9?
Nearly, but not exactly. Grade 8 has a minimum tensile strength of 150 ksi (1 034 MPa) and class 10.9 of 1 040 MPa. The dimensions and tolerances differ, so check the standard for the application.
Sources
- ISO 898-1, Mechanical properties of fasteners made of carbon steel and alloy steel: bolts, screws and studs.
- SAE J429, Mechanical and Material Requirements for Externally Threaded Fasteners; ASTM A307; ISO 3506 (stainless steel fasteners).
- ISO 898-1 (minimum values for carbon and alloy steel bolts, screws and studs; yield is the 0.2 percent proof stress, or the lower yield for classes 4.6 to 5.6), SAE J429 and ASTM A307 (minimum values in ksi). The MPa and ksi columns are converted from the standard values (1 ksi = 6.894757 MPa). Class 4.8 and 5.8 yield values are minimums for the whole size range; check the standard for the exact limits by size.