Specific Gravity

Specific gravity, abbreviated as SG, a dimensionless number, is the density or ratio of any substance to another substance.  It sometimes may be called just gravity or relative density.  When calculating the specific gravity of a liquid or solid, water is normally the comparison (water has a specific gravity of 1).  Even though two objects are the same size, their density may be different.  The density of the substance and the density of water must be measured using the same units of mass and volume for accurate comparison.  This is expressed in how much more weight the object is in comparison to the same amount of water.  Likewise a gas is compared to air.

The specific gravity of a substance is determined by comparing its density to the density of water at a specified temperature and pressure.  By definition, the specific gravity of water is 1.  Therefore, if the specific gravity of a substance is less than 1, it is less dense than water, and if it is greater than 1, it is denser than water.

For example, in the context of minerals and ores, specific gravity is often used to determine the mineral composition and to assess the concentration of valuable components.  In the petroleum industry, specific gravity is used to classify and measure the density of crude oil and other hydrocarbon liquids.  It's important to note that specific gravity is temperature dependent because the density of both the substance and water can vary with temperature.  Therefore, when specific gravity is specified, the temperature at which it is measured should be indicated.

Specific Gravity Formula
\( SG \;=\; \rho_s \;/\; \rho_r  \)     (Specific Gravity) \( \rho_s \;=\; SG \; \rho_w \)  \( \rho_w \;=\; \rho_s \;/\; SG \)
Symbol	English	Metric
\( SG \) = Material Specific Gravity (water = 1)	\(dimensionless\)	\(dimensionless\)
\( \rho_s \)  (Greek symbol rho) = Substance Density	\(lb \;/\; ft^3\)	\(kg \;/\; m^3\)
\( \rho_w \)  (Greek symbol rho) = Reference Density	\(lb \;/\; ft^3\)	\(kg \;/\; m^3\)