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When knowledge of the dielectric properties, or permittivity, of granular or powdered materials is important, as in dielectric heating applications or in the sensing of moisture content by radio-frequency or microwave instruments, some reliable relationship between the dielectric properties and the bulk density of the material is required. Linear relationships between functions of the permittivity and bulk density have been observed, and dielectric mixture equations have been explored to obtain useful relationships for determining the dielectric properties of granular and powdered materials. These findings are reviewed, useful permittivity-density relationships are identified, and an equation is provided that permits reliable calculation of the permittivity of a particulate material at any given bulk density, if the permittivity is known or measured at another known bulk density. The relationships are illustrated for pulverized coal, wheat, and ground wheat. The useful mixture equation is valid for granular and powdered materials with permittivities in the same range as those for grain and coal.