Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is (the lowercase Greek letter rho), although the Latin letter "D" can also be used. Mathematically, density is defined as mass divided by volume. Another term that is commonly used is specific gravity. Specific gravity is the ratio of the density (mass of a unit volume) of a substance to the density of 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, the reference is air at room temperature (20 C or 68 F). A hydrometer is an instrument used for measuring the relative density of liquids based on the concept of buoyancy. The hydrometer makes use of Archimedes' principle: a solid suspended in a fluid is buoyed by a force equal to the weight of the fluid displaced by the submerged part of the suspended solid. The lower the density of the fluid, the deeper a hydrometer of a given weight sinks; the stem is calibrated to give a numerical reading. They are typically calibrated and graduated with one