Bolt grade chart
Mechanical strength of common bolt grades and classes, with the head markings used to identify them. Higher grades carry more load but are less ductile. Use it to match a bolt to the strength a joint needs.
SAE grades (inch bolts)
| Grade | Proof (ksi) | Yield (ksi) | Tensile (ksi) | Head marking |
|---|---|---|---|---|
| Grade 2 | 55 | 57 | 74 | No marks |
| Grade 5 | 85 | 92 | 120 | 3 radial lines |
| Grade 8 | 120 | 130 | 150 | 6 radial lines |
Metric property classes
| Class | Proof (MPa) | Yield (MPa) | Tensile (MPa) | Marking |
|---|---|---|---|---|
| 8.8 | 600 | 660 | 800 | 8.8 |
| 10.9 | 830 | 940 | 1040 | 10.9 |
| 12.9 | 970 | 1100 | 1220 | 12.9 |
Strength values are minimums for common diameters; very large or very small sizes can differ. The metric class encodes strength: the first number is about the tensile strength in hundreds of MPa, and the two multiplied give roughly the yield. Higher grades are stronger but more brittle and notch-sensitive.
Sizing a bolt for a load?
Pair these strengths with the Bolt Torque Calculator to set the right preload for the joint.
Reading bolt grade markings
Inch bolts show their grade as radial lines on the head: none for Grade 2, three for Grade 5, six for Grade 8. Metric bolts are stamped with their class, such as 8.8 or 10.9. The markings let you identify strength at a glance.
Proof, yield, and tensile strength
Proof load is the stress a bolt takes without permanent stretch — the practical working limit. Yield is where it begins to deform permanently, and tensile is where it finally breaks. Preload is normally set as a fraction of proof so the bolt stays elastic.
Bolt head marking identification
The radial lines stamped on a bolt head tell you its SAE grade at a glance. Count the marks:
No marks
3 radial lines
6 radial lines
Grade 2 heads are unmarked. Grade 5 carries three evenly spaced lines, and Grade 8 carries six. Metric bolts instead show a numeric class such as 8.8 or 10.9.
Quick comparison
| Grade | Strength | Tensile (psi) | Common uses |
|---|---|---|---|
| Grade 2 | Low | 74,000 | General hardware, light duty |
| Grade 5 | Medium | 120,000 | Automotive, machinery |
| Grade 8 | High | 150,000 | Heavy equipment, suspension |
FAQ
Is Grade 8 stronger than stainless steel? In raw tensile strength, yes. A Grade 8 bolt is rated near 150,000 psi, while common 18-8 (A2) stainless runs about 70,000 to 100,000 psi. Stainless wins on corrosion resistance, not strength; specialized alloys exist where both are needed.
Can metric and SAE grades be compared? Roughly. Metric class 8.8 is comparable to SAE Grade 5, and class 10.9 is close to SAE Grade 8. They are not identical, so build to the spec your design calls for rather than mixing systems.
What is the difference between Grade 5 and Grade 8 bolts?
Grade 8 is stronger, about 130 ksi yield and 150 ksi tensile versus 92 and 120 for Grade 5. It carries more load but is less ductile.
What does 10.9 mean on a metric bolt?
It is the property class. The 10 indicates about 1000 MPa tensile strength, and 10 times 0.9 gives roughly 900 MPa yield, comparable to an inch Grade 8.
Is a higher grade always better?
Not always. Higher grades are stronger but more brittle and notch-sensitive, and can be less forgiving under shock or fatigue. Match the grade to the load and conditions.