Bolt grade chart

Count the lines and add two - and then stop

Count the radial lines and add two. That rule identifies exactly two grades - SAE Grade 5 with three lines and SAE Grade 8 with six - and it fails everywhere else. Grades 1, 2 and 4 carry no marking at all, so an unmarked head is not evidence of Grade 2; it is evidence of nothing, and unmarked hardware that meets no standard looks exactly the same. ASTM grades are stamped with the specification (A325, A490, BC, BD, 307A) rather than lines, and metric classes are stamped with the class number. Four marking systems share one bin.

Strength is banded by diameter

A grade is not one number. SAE Grade 5 drops from 85 ksi proof to 74 ksi above 1 inch; ASTM A449 drops twice more, to 55 ksi above 1-1/2 inch; ISO class 8.8 steps up at 16 mm rather than down. Every grade page here carries its bands separately, because a single figure per grade is wrong for part of the range it is quoted over.

Inch grades are published in ksi and metric classes in MPa. The other column on every row is a conversion at 1 ksi = 6.894757 MPa, rounded to the nearest whole unit - it is arithmetic on the published figure, not a second published figure.