Editing Density (section)

=== Non-compact materials ===
{{further|Bulk density|Particle mass density}}

In practice, bulk materials such as sugar, sand, or snow contain voids. Many materials exist in nature as flakes, pellets, or granules.

Voids are regions which contain something other than the considered material.  Commonly the void is air, but it could also be vacuum,  liquid, solid, or a different gas or gaseous mixture.

The ''[[bulk volume]]'' of a material —inclusive of the [[void space fraction]]— is often obtained by a simple measurement (e.g. with a calibrated measuring cup) or geometrically from known dimensions.

Mass divided by bulk volume determines ''[[bulk density]]''.  This is not the same thing as the material volumetric mass density.
To determine the material volumetric mass density, one must first discount the volume of the void fraction. Sometimes this can be determined by geometrical reasoning.  For the [[close-packing of equal spheres]] the non-void fraction can be at most about 74%.  It can also be determined empirically.  Some bulk materials, however, such as sand, have a ''variable'' void fraction which depends on how the material is agitated or poured. It might be loose or compact, with more or less air space depending on handling.

In practice, the void fraction is not necessarily air, or even gaseous.  In the case of sand, it could be water, which can be advantageous for measurement as the void fraction for sand saturated in water—once any air bubbles are thoroughly driven out—is potentially more consistent than dry sand measured with an air void.

In the case of non-compact materials, one must also take care in determining the mass of the material sample. If the material is under pressure (commonly ambient air pressure at the earth's surface) the determination of mass from a measured sample weight might need to account for buoyancy effects due to the density of the void constituent, depending on how the measurement was conducted. In the case of dry sand, sand is so much denser than air that the buoyancy effect is commonly neglected (less than one part in one thousand).

Mass change upon displacing one void material with another while maintaining constant volume can be used to estimate the void fraction, if the difference in density of the two voids materials is reliably known.