How to choose the right bolt grade
Bolt grades tell you how much load a fastener can take before it yields. The two systems you will meet most are metric (8.8, 10.9, 12.9) and SAE (Grade 5, Grade 8).
8.8 is the everyday workhorse for machinery and construction. 10.9 steps up tensile strength by roughly 30% and suits suspension and press frames. 12.9 is alloy steel for high-stress, precision applications — but it is more brittle, so avoid it where shock loads dominate.
Rule of thumb: match the grade to the load, then match the coating to the environment. A 10.9 bolt with plain finish will rust long before it fails mechanically outdoors.