How to find the overall visible area of overlapping 3 or more Octagons? I have 3 Octagons of same size. I know the coordinates of their centers and their side length. How can I calculate the overall area? (Shaded area in the image)

In the image above, the area comes out to be 1343.73 sq units, calculated via a CAD software. I'd like to know how to calculate that manually? I need to make an algorithm.

You have enough information to find the vertices of each octagon.
A search for overlapping polygons area finds many links to versions of the the polygon clipping problem.   Something there might solve your problem.
https://www.google.com/search?client=ubuntu&channel=fs&q=overlapping+polygons+area&ie=utf-8&oe=utf-8
