What Is Specific Gravity of Soil Solids? Specific gravity of soil solids (Gs) is the ratio of the mass of a given volume of soil solids to the mass of an equal volume of water at the same temperature. Gs = Mass of Soil Solids / Mass of Equal Volume of Water Physically, it describes how dense the soil mineral particles are relative to water. A Gs value of 2.65 means the soil grains are 2.65 times heavier than an equal volume of water. This value depends entirely on the mineral composition of the soil particles themselves — not on how densely the soil is packed or how much water it contains. That distinction is important. Void ratio, density, and water content describe the arrangement of soil particles. Gs describes the mineral nature of the solid phase itself. Why Is Gs Usually Taken as 2.65? The reason 2.65 is so widely used is straightforward: quartz (silicon dioxide, SiO₂) is the dominant mineral in many natural soils encountered in civil engineering. Natural sands, silts derived from sandstone weathering, and many residual soils contain high percentages of quartz minerals. Since quartz has a specific gravity close to 2.65, soils dominated by quartz particles tend to cluster around that value as well. Many of the classic soil mechanics experiments and laboratory procedures were originally developed using quartz-rich sands. Over time, the value became deeply embedded in textbooks, laboratory manuals, and engineering practice. For silica-dominant soils, using 2.65 is generally a reasonable approximation.