Formula: A formula is nothing but a relation between two or more variables.
Subject of Formula: The subject of a formula is a variable which is expressed in terms of other variables.
Some commonly used formulas are:
Perimeter of a square of side (a):
Area of a square of side (a): 

Volume of a cube of side (a):  
Perimeter rectangle of width ( w ) and length (l):
Area of a rectangle of width ( w ) and length (l): 

Volume of a cuboid of width (a), length (b) & height (h):  
Circumference of a circle of radius (r):
Area of a circle of radius (r): 

Surface area of a sphere of radius (r):
Volume of a sphere of radius (r): 

Surface area of a cone of radius (r) and height (h):
Volume of a cone of radius (r) and height (h): 

Surface area of a cylinder of radius (r) and height (h):
Volume of a cylinder of radius (r) and height (h): 
But then the formula could be any relation between variables. For example, the newton’s laws of motion as stated below
or something as simple like