Specific gravity is a dimensionless quantity, meaning it has no unit. It is defined as the ratio of the density of a substance to the density of a reference substance, typically water for liquids and solids, and air for gases. Since specific gravity is a ratio of two densities, the units cancel out, leaving it without any unit.
For example, if you have a liquid with a density of 1.2 g/cm³ and the density of water is 1 g/cm³, the specific gravity of the liquid would be:
Specific Gravity = Density of Substance / Density of Water
Specific Gravity = 1.2 g/cm³ / 1 g/cm³ = 1.2
As you can see, the units (g/cm³) cancel out, resulting in a unitless value of 1.2. This value indicates that the liquid is 1.2 times denser than water.