text: For a fully filled duct or pipe whose cross-section is a regular polygon, the hydraulic diameter is equivalent to the diameter  of a circle inscribed within the wetted perimeter.
  This can be seen as follows: The -sided regular polygon is a union of  triangles, each of height  and base .
  Each such triangle contributes  to the total area and  to the total perimeter, giving
  for the hydraulic diameter.