Perimeter
The perimeter of a polygon (or any other closed curve, such as a circle) is the distance around the outside.
Here are some formulas for the perimeters of common plane figures.


Shape

Formula

Variables

Square

$P=4s$

$s$ is the length of the side of the square. 
Rectangle

$P=2L+2W$

$L$ and $W$ are the lengths of the rectangle's sides (length and width). 
Triangle

$a+b+c$

$a,b$ , and $c$ are the side lengths. 
$P=a+b+\sqrt{{a}^{2}+{b}^{2}}$

$a$ and $b$ are the lengths of the two legs of the triangle  
Circle


$r$ is the radius and $d$ is the diameter. 