Side-by-side torque and strength comparisons to help you choose the right bolt grade for your application.
The two most common high-strength SAE grades
Standard hardware vs structural grade
Metric structural vs high-strength
High-strength vs highest standard class
Basic hardware vs structural grade
Mid-range vs structural standard
System comparison and equivalences
Every comparison on this site holds size, thread and friction constant and changes only the grade, so the torque difference you see is the difference in proof stress. Because torque is calculated as T = K × D × F and F is 75% of proof load, the percentage gap in torque equals the percentage gap in proof stress: class 10.9 (830 MPa) runs 43% above 8.8 (580 MPa), and SAE Grade 8 (120 ksi) runs 41% above Grade 5 (85 ksi).
Two things break that simple ratio. Proof stress steps down with diameter in both standards — ISO 898-1 class 8.8 rises to 600 MPa above M16, SAE Grade 5 falls to 74 ksi above 1" and Grade 2 to 33 ksi above 3/4" — so the gap between grades changes across the size range. And a higher grade only helps if the nut, washer and tapped thread can carry the extra load; a 12.9 bolt in a soft aluminium thread will strip the thread long before it reaches its own torque.
If you are replacing a fastener, match or exceed the original grade and use the torque for the grade you actually install. The metric vs imperial page lists the nearest equivalents between the two systems.