If your diagonals are equal, you are square.

There are some of the most skillful artists in building I've ever known , right here.

At 19 feet 9 inches x 19 feet 9 inches you will be 27 feet 11 3/16 inch diagonl .

Making the calculated dimensions just give a starting point. Once you get your anchor point just tweek the other three corner posts to get as close as you can to your calculated dimension. Not rocket science just take your time. You probably won't get it perfect, but it should come out pretty darn close. Building a pole barn for the first time is a lot of work but when done is a thing of beauty. Three years ago my son and I built his 30' x 40'. Biggest hassle was auguring the post holes, lots of good to big rocks, and then jockeying the 8x8x20 posts. those suckers are real heavy.

I wouldn't believe this guy if he said the earth was round !

Thanks for all the help guys next problem is finding treated lumber. Seems to be a shortage around here. I had to order longer lengths and hope they get shipment in tomorrow to finish the rest of my order.

There is a shortage of treated lumber everywhere right now. Been that way for a month or more. I was at my local lumberyard to pick up some 2 x 12 x 8 treated to build a deck steps for a customer. All they had was a handful of 2 x 12 x 10’s. They gave them to me for the 2 x 8 x 12 price.

For large measuring like yours I use the 3/4/5 triangle. A triangle with one leg any unit of 3 with the other leg the same unit of 4 with a Hyp of 5 will have a 90 degree(right angle) between legs. I've laid out large rectangles using long 1x4's or even rope. The woods costs a little but when screwed into magic triangles can be placed at each corner and moved around easier until all squared up. I make 15'x12'x 9'. Haven't yet not found use for the 1x4's reused in the project so no money lost on them.

Cheapo Depot can special order lumber up to 26'.

If it is going to be steel sided, I would make it 21x21

I built mine to those same dimensions and started out with 9 posts, also. I ended up taking the middle post out; just in the way.

Ha! I was leaving an allowance for the width of the steel siding!

ol'dad