Specific Gravity in the Imperial system is equivalent to what in the metric system?

Specific gravity is defined as the ratio of the density of a substance to the density of a reference substance, typically water for liquids, at a specific temperature. In the Imperial system, specific gravity and relative density essentially mean the same thing. They both compare the density of a substance to that of water and are dimensionless because they are ratios of similar units.

The term "relative density" is commonly used in scientific contexts to describe this same concept, making it the correct counterpart in the metric system. This metric terminology emphasizes the relationship between the substance's density and the reference density, without needing to specify units, as both densities cancel out.

The other terms do not appropriately match the definition or implication of specific gravity. “Relative gravity” and “weight density” relate to different concepts involving gravity or the weight of substances rather than density comparisons, while “specific density” is not a standard term recognized in either the metric or Imperial context for this concept. Therefore, understanding these definitions confirms that the appropriate metric equivalent of specific gravity is indeed relative density.