Relative Density

Relative density, abbreviated as RD or \(D_r\), a dimensionless number, also called the coefficient of relative density or specific gravity, used to express the density of a material or substance relative to the density of another reference material or substance.  Traditionally, specific gravity is used to compare the density of a substance relative to air (SG=1) or water (also SG=1).

In geotechnical engineering, relative density is often used to describe the compaction or density of granular soils, such as sands and gravels.  It is defined as the ratio of the difference between the maximum and minimum void ratios of a soil to the difference between the void ratios of the same soil under fully saturated and completely dry conditions.

In geotechnical engineering, relative density is commonly used to classify soils and assess their engineering behavior, including compaction characteristics, shear strength, and settlement potential.  It provides insights into the density and arrangement of soil particles, which influences the soil's mechanical properties and response to applied loads.  In other fields, such as materials science and fluid dynamics, relative density can refer to the ratio of the density of a substance to the density of a reference substance, which is typically water.  This value is also referred to as specific gravity and is often used for comparing the densities of liquids and solids.

Relative Density Formula
\( D_r \;=\; \dfrac{ \rho_s }{ \rho_r }\)     (Relative Density) \( \rho_s \;=\;  D_r \cdot \rho_r  \) \( \rho_r \;=\; \dfrac{ \rho_s }{ D_r } \)
Symbol	English	Metric
\( D_r \) = Relative Density	\(dimensionless\)	\(dimensionless\)
\( \rho_s \)   (Greek symbol rho) = Substance Density	\(lbm \;/\; ft^3\)	\(kg \;/\; m^3\)
\( \rho_r \)   (Greek symbol rho) = Reference Density	\(lbm \;/\; ft^3\)	\(kg \;/\; m^3\)