The specific gravity of mercury is 13.6. Specific gravity is also called relative density and is the ratio of the density of one substance to that of a standard substance such as water. Density is an object's mass per unit volume.
Mass is a measure of the amount of matter in a substance (informally weight, although weight implies the presence of gravity). The standard substance most commonly used to compare for specific gravity is water at 4 degrees Celsius, which has a density of 1 kilogram per liter. Specific gravity has no dimension because it is a ratio of two quantities with the same dimension (mass per unit volume).