A pentagon has five side and five angles. A polygon's diagonals are line segments from one corner to another, but not the sides. The diagonals of a convex regular pentagon are in the golden ratio to its sides. Below given Pentagon Diagonal Length Calculator calculates the diagonal length of a five-sided pentagon given the value of a side. Just input the side of the pentagon and the calculator will automatically update you with the diagonal value.
A pentagon with one of its side having a length of 12 cm
Pentagon Diagonal Length = (1+ √5) / 2) x 12
= 19.4164 cm