Whether designing in inch or metric fasteners, you need to be aware of the grade (inch) or property class (metric) of your fasteners. Below is a matrix that helps you compare the difference between the two:
When designing with nuts and bolts, grades and property classes of hardware should match. However, higher strength nuts may be used with lower strength bolts. The nut must always be stronger than the bolt for safe and proper joint design, so using a lower strength nut with a higher strength bolt is asking for trouble.Typically, as strength increases, so does price. It is also important to note that stronger is not always better. High strength fasteners tend to be less flexible and often require better tightening control to take advantage of their strength.
Finally, strength of washers is often overlooked. The purpose of using a flat washer under a hex head bolt or nut is to spread the load and increase the surface pressure limit of the bearing surface. If non-hardened washers are used with hardened bolts, the hex head can dig into the face of the washer, allowing the clamp load to settle and the joint to loosen.