Metric vs SAE Bolt Grades
Metric vs SAE Bolt Grades
Conversion chart, strength comparisons, and why they're not exact matches
Working on a project with mixed fasteners? Need to substitute metric for SAE (or vice versa)? This guide breaks down the equivalents—and explains why they're never quite exact matches.
Quick Cross-Reference Chart
| SAE Grade | Metric Class | Tensile Strength | Match? |
|---|---|---|---|
| Grade 2 | Class 4.6 / 4.8 | ~74 ksi / ~400 MPa | Close |
| Grade 5 | Class 8.8 | ~120 ksi / ~800 MPa | Close |
| — | Class 9.8 | ~131 ksi / ~900 MPa | Between 5 & 8 |
| Grade 8 | Class 10.9 | ~150 ksi / ~1040 MPa | Close |
| Grade 9 | Class 12.9 | ~174-180 ksi / ~1220 MPa | Close |
The Two Most Important Equivalents:
- Grade 5 ≈ Class 8.8 — Both ~120 ksi tensile
- Grade 8 ≈ Class 10.9 — Both ~150 ksi tensile
Detailed Strength Comparison
Grade 5 vs Class 8.8
| Property | SAE Grade 5 | ISO Class 8.8 | Verdict |
|---|---|---|---|
| Tensile Strength | 120,000 PSI (827 MPa) | 116,000 PSI (800 MPa) | Grade 5 slightly stronger |
| Yield Strength | 92,000 PSI (634 MPa) | 91,000 PSI (640 MPa) | Nearly identical |
| Proof Load | 85,000 PSI (586 MPa) | 84,000 PSI (580 MPa) | Nearly identical |
| Heat Treatment | Quenched & tempered | Quenched & tempered | Same |
Practical conclusion: For most purposes, Grade 5 and Class 8.8 are interchangeable by strength. The difference is threads—you can't screw a metric bolt into an SAE nut.
Grade 8 vs Class 10.9
| Property | SAE Grade 8 | ISO Class 10.9 | Verdict |
|---|---|---|---|
| Tensile Strength | 150,000 PSI (1,034 MPa) | 150,000 PSI (1,040 MPa) | Essentially equal |
| Yield Strength | 130,000 PSI (896 MPa) | 122,000 PSI (940 MPa) | Grade 8 slightly higher |
| Proof Load | 120,000 PSI (827 MPa) | 120,000 PSI (830 MPa) | Essentially equal |
Practical conclusion: Grade 8 and Class 10.9 are effectively equivalent in strength. Use whichever matches your thread system.
Class 12.9 is NOT equivalent to Grade 8—it's 16% stronger (174 ksi vs 150 ksi). It's closer to Grade 9. Don't substitute Grade 8 where 12.9 is specified.
Why They're Not Exact Equivalents
Different Standards
SAE J429 (US inch-series) and ISO 898-1 (metric) were developed independently with different test methods and tolerances.
Thread Pitch Affects Strength
Different thread pitches create different stress areas. The actual load capacity varies slightly even with identical material strength.
Material Chemistry
While both specify "medium carbon steel," the exact chemistry ranges differ between standards.
Testing Methods
SAE tests full-size bolts in specific ranges; ISO uses calculated values from material properties.
The Size Problem
The real barrier to substitution usually isn't strength—it's dimensions. No metric size exactly equals common SAE sizes:
| SAE Size | Closest Metric | Difference |
|---|---|---|
| ¼" (0.250") | M6 (0.236") | 6% smaller |
| 5/16" (0.3125") | M8 (0.315") | Close |
| ⅜" (0.375") | M10 (0.394") | M10 larger |
| ½" (0.500") | M12 (0.472") | M12 smaller |
| ⅝" (0.625") | M16 (0.630") | Close |
| ¾" (0.750") | M20 (0.787") | M20 larger |
Can You Substitute?
✅ Yes, If:
- Strength is equivalent or higher
- Thread matches (non-negotiable)
- Same diameter and length
- Application isn't spec-controlled
❌ No, If:
- Spec requires a specific grade
- Thread pitch doesn't match
- Certifications/traceability required
- Strength is lower
Complete Cross-Reference Table
| SAE Grade | Head Marks | Tensile (PSI) | ≈ Metric Class | Head Marks | Tensile (MPa) |
|---|---|---|---|---|---|
| Grade 2 | None | 74,000 | Class 4.8 | "4.8" | 400 |
| Grade 5 | 3 lines | 120,000 | Class 8.8 | "8.8" | 800 |
| — | — | — | Class 9.8 | "9.8" | 900 |
| Grade 8 | 6 lines | 150,000 | Class 10.9 | "10.9" | 1,040 |
| Grade 9 | 6 lines + "9" | 180,000 | Class 12.9 | "12.9" | 1,220 |
Frequently Asked Questions
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