When you have finished this page, try the Perimeter Quiz.
The perimeter of any polygon is the measurement of the distance around the outside of the polygon or circle. The formulas or equations differ depending on the polygon. In all cases save a circle, one can simply add up the measure of the length of each of the sides of a polygon to get the perimeter.
5 Units in Length  
5 Units in Length  5 U
nits in Length 

5 Units in Length 
The perimeter of a square would be the sum of the lengths of the sides of the square. Since all sides of a square are equal in length, the perimeter would be 4 times the length of one side of the square. In the example above, the perimeter would be 20 units.
The perimeter of any equilateral or equal sided polygon is the number of sides of that polygon times the length of one side. The chart below:
Polygon  Perimeter 
Equilateral Triangle
Length of side times three 

Equilateral Quadrilateral (Square or Rhombus)
Length of side times four 

Equilateral Pentagon
Length of side times five 

Equilateral Hexagon
Length of side times six 

Equilateral Heptagon
Length of side times seven 

Equilateral Octagon
Length of side times eight 

Equilateral Nonagon
Length of side times nine 

Equilateral Decagon
Length of side times ten 

Equilateral Decagon
Length of side times twelve 
To get the perimeter of a circle, called the circumference of the circle, one simply has to multiply 2 times pi (about 3.14) times the radius of the particular circle.
To get the perimeter of a rectangle, one only needs to know the length of one side and the width of one side of the rectangle. Thus the formula for the perimeter of a rectangle would be:
Perimeter = 2 times the length plus two times the width
P = 2L + 2W
or Perimeter = 2 times the length plus the width.
P = 2(L+W)