Archimedes principle:

When an object is immersed partially or fully in a fluid, it experiences a loss of weight which is equal to the weight of fluid displaced. The formula for relative density can be written in terms of weight as;

Relative density = weight of the object in water ----------------1.2
Weight of an equal volume of water

Weight of an equal volume of water is the same as the loss of weight of the object when completely immersed in water. Using this, the formula can be written as

Relative density = weight of the object in air --------------------1.3
Its loss of weight in water
In order to calculate the relative density of a liquid, the loss of weight of an object in the liquid as well as in water is calculated so that;
Relative density of the liquid = loss of weight of the object in liquid 1.4
Its loss of weight in water