# How to Calculate Relative Density

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

$D_r = \frac{\rho_s}{\rho_{ref}}$
Where:
Dr = Relative Density (dimensionless)
ρs = bulk density of soil (kg/m3 or lb/ft3)
ρref = bulk density of reference (kg/m3 or lb/ft3)

Often, the reference density is that of water, 62.4 lbs/ft3 or 1000 kg/m3.

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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