the ratio of the density of a substance to the density of a reference substance. Specific Gravity does not have any units. When water is used as the reference material, specific gravity equals density (without units). 

Specific Gravity (SG) = (Density of substance g/ml)/(Density of reference substance g/ml)
Notice that g/ml cancels out so that there are no units associated with specific gravity. Specific Gravity then becomes an abstract number.