Perimeter is the sum of the sides of a polygon. It is a distance and therefore is a one-dimensional property. The perimeter of a circle is called its circumference, and can be found using the formula Circumference = 2(pi)(radius). For a square, the perimeter = 4(side length), and for a rectangle, perimeter = 2(length of the width) + 2(length of the height).
A term that we use in geometry that you've definitely seen sometime in 6th, 7th, 8th, grade sometime before Geometry is parameter. And the parameter is just the sum of the lengths of the sides of a polygon. So if we talk about a square where are the sides are congruent the parameter is just calculated by saying four times S where S is your side length. If you have a rectangle where you have opposite sides congruent you're going to have two bases. So we are going to say two times B, where B is one of your bases, plus two of your heights or two times H. In a circle we don't call it perimeter, we call it circumference. So the circumference is essentially the perimeter of a circle. And there you only need to know your radius. That is going to be equal to two times pi times your radius. So to calculate your parameter of any polygon just add up the sides. Specifically, for a square you can use the shortcut of four times the side length, for a rectangle you can use two times the base plus two times the height, and for a circle you can two times pi times the radius.