15 posts plus 2 corner posts = 17 posts. With 17 posts there would be 16 spaces each side.
The space between each is 8 1/8 feet.
The length of one side is the number of spaces multiplied by the distance between each space.
Total length of one side = 16 spaces x 8 1/8 feet = 130 feet
Area = length x width.
It’s a square shape so area = 130 x 130 = 16,900 square feet
Perimeter is the distance around the square, this would be the length of one side x the number of sides.
Perimeter = 130 x 4 = 520 feet