What is the S.I unit of relative density?

The S.I unit of relative density is "has no S.I unit." Relative density, also known as specific gravity, is the ratio of the density of a substance to the density of a reference substance, usually water. Since it is a ratio of two quantities with the same unit, it does not have a unit itself. Therefore, relative density has no S.I unit. For example, if the density of a substance is 5 g/cm^3 and the density of water is 1 g/cm^3, then the relative density of the substance is 5/1 = 5. Note that the units of both densities cancel out, leaving a dimensionless quantity. In summary, relative density is a dimensionless quantity, which means it has no S.I unit.