This page has stuff to help with unit conversion.  Eventually it will also have a bunch of materials that you can add to your .ini file to customize your mass calculations.

[[http://www.gordonengland.co.uk/conversion/density.htm|Unit converter]]

[[http://www.tesarta.com/www/resources/library/weights.html|Weights of common substances (use the numbers under the specific gravity heading)]]

[[http://www.plasticsusa.com/specgrav2.html|Specific gravities of plastics]]

[[http://www.reade.com/Particle_Briefings/spec_gra.html|Specific gravities of metals, ceramics, and minerals]]