Understanding the differences between ISO metric and SAE/ASTM imperial bolt standards, grade equivalences, and when each system applies.
Metric property classes and SAE grades are different systems that don't map perfectly, but these are the closest equivalents based on proof strength:
| Metric Class | SAE Grade | Proof Strength | Typical Application |
|---|---|---|---|
| 4.6 / 4.8 | Grade 2 | 225–310 MPa / 33 ksi | Light duty, non-structural |
| 8.8 | Grade 5 | 580 MPa / 85 ksi | General structural, automotive |
| 10.9 | Grade 8 | 830 MPa / 120 ksi | High-strength, heavy equipment |
| 12.9 | ASTM A574 | 970 MPa / 140 ksi | Socket head cap screws |
| Metric | Imperial | Diameter |
|---|---|---|
| M5 | #10 | 5.0 mm vs 4.83 mm |
| M6 | 1/4" | 6.0 mm vs 6.35 mm |
| M8 | 5/16" | 8.0 mm vs 7.94 mm |
| M10 | 3/8" | 10.0 mm vs 9.53 mm |
| M12 | 1/2" | 12.0 mm vs 12.70 mm |
| M16 | 5/8" | 16.0 mm vs 15.88 mm |
| M20 | 3/4" | 20.0 mm vs 19.05 mm |
These are not exact substitutions. Metric and imperial threads have different pitch, form, and stress areas. Never interchange without verifying thread compatibility.
Thread compatibility: Metric and imperial threads are NOT interchangeable, even at similar diameters. An M12 bolt will not thread into a 1/2"-13 nut. Attempting to force cross-system compatibility will strip threads and compromise the joint.
Torque units: Metric specifications use N·m (newton-meters). Imperial specifications commonly use ft·lbs (foot-pounds). The conversion factor is 1 ft·lb = 1.3558 N·m. TorqueSpec provides both units on every page.
Global trend: Most of the world uses metric exclusively. The US, and some legacy equipment worldwide, still uses imperial. New designs are increasingly metric-only, but maintenance and aftermarket work frequently requires both systems.