 The area of a regular tetracontagon is (with )

and its inradius is

The factor formula_3 is a root of the octic equation formula_4.The circumradius of a regular tetracontagon is

As 40 = 23 × 5, a regular tetracontagon is constructible using a compass and straightedge.As a truncated icosagon, it can be constructed by an edge-bisection of a regular icosagon.