What is Perimeter?
In geometry, perimeter can be defined as the path or the boundary that surrounds a shape. It can also be defined as the length of the outline of a shape.
We often find the perimeter when putting up Christmas lights around the house or fencing the backyard garden. Other examples may include finding the total length of the boundary of the soccer field or the length of the crochet or ribbon required to cover the border of a table mat.
How to find perimeter?
For small irregular shapes, we can use a string of thread and place it exactly along the boundary of the shape, once. The total length of the string used along the boundary is the perimeter of the shape.
The perimeter of all polygons can be determined by adding the lengths of their sides/edges.
Regular Polygons  Irregular Polygons 


Here’s how the perimeter of some common shapes is calculated:
Polygon name :  Polygon Picture/Image :  Perimeter Formula : 
Equilateral Triangle  P = 3 x a  
Scalene Triangle  P = a + b + c  
Square  P = 4 x a  
Rectangle  P = 2 ( a + b )  
Quadrilateral  P = a + b + c + d  
Regular Pentagon  P = 5 x a  
Regular Hexagon  P = 6 x a  
Regular Octagon  P = 8 x a  
Regular Ngon  A regular ngon with each side a units long 
P = n x a , where n is the number of sides and a is the lenght of the sides. 