Polygon with twelve sides.
 A dodecagon is a regular dodecagon if all its sides are congruent and if all its interior angles are congruent.
 A dodecagon has twelve interior angles; if it is a regular dodecagon, each of its interior angles measures 150°.
Formula
The formula to calculate the area A of a regular dodecagon of side length c is:
\(\mathit{A}=3c^{2}(2+\sqrt{3})\)
Examples
 The figure below is a regular dodecagon. The segment a represents its apothem.

 This is an irregular and nonconvex dodecagon: