What is the unit of relative density?

Relative density, also known as specific gravity, is a dimensionless quantity, meaning it does not have any units. It is defined as the ratio of the density of a substance to the density of a reference substance, typically water for liquids and air for gases. Since it is a ratio of two densities, any units used will cancel each other out. Therefore, it is not expressed in units like kg/m³, lbs/ft³, or N/m², which measure density, force, or pressure. This is why "none" accurately reflects the nature of the unit for relative density.