One of the very powerful aspects of Inventor is to use the Parametrers list. Math is incorporated here. It is also where dimensions are renamed to meaningful words. The picture below shows the parameters list. In this example, the parameters for length, and Width, and Height have been renamed. A Parameter has been added that is the material density. And another Parameter has been added called the weight. It is computed and displayed. It is also check marked for export. (See the checkmark to the right) By checking this, the value becomes available for use in the drawing ab an item in the title block, or as a sketched symbol, for example. See Tip # 2 for implementation as a sketched symbol.
Note that in this case I have created the Raw Part Weight to place elsewhere. The math was done here, based on the part geometry before other cuts were made. The check mark in the Parameters list makes that value exportable.