How to handle Plates in Item Master

As currently in excel sheet we are using 

Plates – Length * Width * Thickness * Density  Qty is tracked in nos and while purchasing in tons. And for each Plates we are assigning a Unique QC tag no. 

And also when we are getting the reminant plates from the parent item we will create a new item for balance material but the QC tag assigned will remain constant. At the same time the purchase is purchasing the plates on the basis of weight. 

So I want to know how can we create a item and what will be the UOM. Can u provide some examples