I have no idea what math needs to be done to figure this out, but let me set the stage for you.
I'm building a BIG planter box in my yard. 27ft X 54ft X 5ft high. We want to utilize our whole yard to grow in, and as it stands, we just have way too much standing water/flooding to do it in the ground without raising it up.
I've gone through a few different ideas of the best way to do it, but I've settled on this: I'm using reclaimed 2x4s from pallets to build wooden walls, which I then will screw into posts around the perimeter of the box. Trying to save as much money as possible, since I simply can't afford all the supplies if I were to buy them new.
That bears the question, just how many posts do I need? I was planning on buying 6X6 posts, but using 4X4s would be cheaper if I could get away with it. I know I need at least a post on each corner, and probably some unknown amount of posts in between the corners. Factoring in concrete needed to support the post, each 6X6 post would cost about $70, and each 4X4 would cost about $50. I appreciate help figuring this out, more so if I can see some sources on how to calculate this myself. Thanks in advance!