There is a formula you can use to find the number of diagonals for any polygon. It is n(n-3)/2, where n is the number of sides. So for a quadrilateral it would be 4(4-3)/2=2 diagonals, a pentagon or would be 5(5-3)/2=5, a hexagon 6(6-3)/2=9, and an octagon 8(8-3)/2=20.
