This calculator works only for regular polygons - those polygons which have ALL sides equal & ALL interior angles equal.
The above figure shows a regular octagon.
The outer circle surrounding it is called a circumscribed circle (or circumcircle) and the inner circle which is surrounded by the octagon is called the inscribed circle (or incircle).
Angle EAB is one of the 8 interior angles.
Angle EAF is one of the 8 exterior angles and equals central angle ACB.
Line AC is called the circumradius (the radius of the outer, cirumscribed circle).
Line CD is called the inradius (the radius of the inscribed circle).