The area of a rectangle is length times width. Since a square's length is equal to its width, a square's area is equal to the length of one side times itself. So, reversed, the square root of the square's area gives the length of one side. In this case, the square root of 36 square centimeters is 6 centimeters. Again, since all four sides of a square are the same, its perimeter is equal to the length of one side times four. For a square with one side equal to 6 centimeters, the perimeter equals 24 centimeters.