Relative density is the density of an object compared to the density of water(which is 1kg/m^3). Relative density doesn't have a unit. It is a method for comparing the densities. Sometimes if the respective object's density is given for comparison then its value will be in the denominator. It is calculated as:

The Density of the given object/ the density of the respective object (Usually water)

The relative density of the object may be defined as it weight in air Wa,divided by the difference of its weight in the air and its wight wen submerged in water,Ww.
for a lead weight,Wa=1.097Ww