The true density of a powder is defined as the ratio of its mass to its true volume, excluding the interparticle void spaces.

In this method, a pycnometer or specific gravity bottle (25 mL) is used. A weighed amount of powder is introduced into the bottle, and a non-solvent liquid (one in which the powder is insoluble) is added to fill the remaining voids. The difference in weights with and without the powder allows calculation of the volume displaced by the powder.

True Density (ρₜ) = Weight of Powder / Volume of Liquid Displaced by Powder

When using liquids such as water or ethanol, the measured density may be slightly less than the true value because these liquids cannot penetrate the smallest voids within the particles.