{kg·m3/kg·m3 g·cm3/g·cm3, lb·ft3/lb·ft3} {Water , Air} Test Process that measures the relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest (at 4 °C or 39.2 °F); for gases, air at room temperature (20 °C or 68 °F) is the reference. The term “relative density” is often preferred in scientific usage. It is defined as a ratio of the density of a particular substance with that of water.