Diagonals of Polygons



square has 2 diagonals octagon has 20 diagonals
A square has
2 diagonals
An octagon has
20 diagonals

A polygon's diagonals are line segments from one corner to another (but not the edges).


The number of diagonals of an n-sided polygon is:

n(n − 3) / 2



Try it Yourself:


polygon concave diagonals A diagonal can actually be outside the polygon,
which happens with some concave polygons.