What is Lever Rule ?

The rule which helps to calculate the relative proportions of liquid phase and solid phase present in the binary mixture at any given temperature is known as Lever rule.

Let, composition of liquid phase is C_L, composition of solid phase is C_S and the composition of (S + L) is C_o in the binary mixture. Then -
Fraction of Solid Phase = (C_L − C_o)/(C_L − C_S)
Fraction of Liquid Phase = (Co − C_L)/(C_L − C_S)
From these formula we can calculate the composition of liquid and solid phase in two phase system using phase diagram.

Let us assume we have a binary mixture (alloy) system where the phase diagram shows:
Composition of C_S = 10% alloy
Composition of C_L = 40% alloy
The overall (S + L) composition C_o = 25% alloy

Applying the Lever Rule:
Fraction of C_S = (C_L − C_o)/(C_L − C_S)
or, Fraction of C_S = (40 − 25)/(40 − 10) = 0.5
So, C_S constitutes 50% of the total.
Similarly, Fraction of C_L = (C_o − C_L)/(C_L − C_S)
or, Fraction of C_S = (25 − 10)/(40 − 10) = 0.5
So, C_L constitutes 50% of the total.