Density is the weight per unit volume of material at 23˚C. Datasheets tend to show the Specific Gravity of a particular material, this is the ratio of the weight of a given volume of a certain material to that of the same volume of water at a given temperature.
Therefore, multiplying the volume of a part in cm3 by the specific gravity will give the weight of that part in grams when moulded in the selected material.
The two test standards that apply here are ASTM D792 and ISO 1183. They differ as shown in the tables below: