* Relative Density* is the ratio of the density of a substance to a specified reference density.

*Where:*

*D _{r} = Relative Density (dimensionless)*

*ρ*

_{s}= bulk density of soil (kg/m^{3}or lb/ft^{3})*ρ*

_{ref}= bulk density of reference (kg/m^{3}or lb/ft^{3})Often, the reference density is that of water, 62.4 lbs/ft^{3} or 1000 kg/m^{3}.

Alternatively, the equation for relative density can use weights instead of densities.

## Relative Density vs. Specific Gravity

Specific Gravity is related to Relative Density in that it compares the density of a substance to that of water instead of a reference density. In some industries there is a reason to use a certain reference density, but most of the time they are used interchangeably as the only reference you would want to use is water.

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