What is the actual square footage of a 12' octagon?
I am going to assume that you have a regular octagon and that the width is 12 feet (w in the diagram below). I am going to think of this octagon as a square with the four corners cut off as in the diagram.
The four pieces cut off can be arranged into two squares with sides of length b feet and hence the area of the octagon is
From the diagram
Hence the area is