The S.I. unit of relative density is 

CONCEPT:

• Density (ρ): Mass per unit volume of an object is known as its density. 

\(ρ = \frac{m}{V}\)

ρ is density, m is mass, and v is volume. 

• Unit of Density = Unit of Mass / Unit of Volume = kg m^-3
• Relative Density (R.D): The ratio of the density of an object with respect to the density of water is known as relative density. 

\(R.D= \frac{ρ '}{ρ}\)

 ρ' is the density of the object,  ρ is the density of the liquid. 

EXPLANATION:

• Unit of Relative Density = (Unit of Density of Object) / (Unit of Density of water)

 (kg m-3) / (kg m-3)  = 1

• So, Relative Density has no unit, because it is the ratio of two similar physical quantities that is density. So, units get canceled out. 

• Students might get mistaken by writing unit of Density instead of relative density.