# Relative Density

Written by Jerry Ratzlaff on . Posted in Dimensionless Numbers

Relative density, abbreviated as RD, is the density or ratio of any substance relative to another substance.  It sometimes may be called just gravity or specific gravity.  Traditionally, specific gravity is used to compare the density of a substance relative to air (SG=1) or water (also SG=1).  The calculation for relative density and the equations used to calculate it are shown below.

 $$\large{ RD = \frac{\rho_s}{\rho_r} }$$          Where: $$\large{ RD }$$ = relative density $$\large{ \rho_r }$$   (Greek symbol rho) = reference density $$\large{ \rho_s }$$   (Greek symbol rho) = substance density