But squared is to the second power. In other words 3.14(3"X3") or 28.26".
I don't think this is quite the right way to approach duct work anyway. Only if your trying to figure square to round as far as what's comparable. In other words a 10" round duct is 79sq. inches. An 8X10 square is 80 sq inches. I know duct guys look at tonage charts and alot of other stuff I don't really know much about when sizing ducts and runs. My guess is this guys problem is running such long runs of big duct work. I don't know how big the rooms are, but off the top of my head 4-6" ducts should be able to get enough flow from an 8X12 supply. By staying large all the way down (56'??) the air isn't getting pushed. I believe you need to downsize the supply trunk as you get further down the line to keep things pressurized. Same way if one of those 6" lines was very long you might start off with an 8" and somewhere along the line drop down to a 6".