Octagon

An octagon is an 8-sided polygon (a flat shape with straight sides).

Regular or Irregular

When all angles are equal and all sides are equal it is regular, otherwise it is irregular

 
Regular Octagon Irregular Octagons

Concave or Convex

A convex octagon has no angles pointing inwards. More precisely, no internal angles can be more than 180°.

When any internal angle is greater than 180° it is concave. (Think: concave has a "cave" in it)

Convex Octagon Concave Octagon

Is it an Octagon?

An octagon has 8 straight sides. The shape must also be closed (all the lines connect up):

Octagon
(straight sides)
Not an Octagon
(has a curve)
Not an Octagon
(open, not closed)

Properties

A regular octagon has:

  • Interior Angles of 135°
  • Exterior Angles of 45°
  • Area = 2(1+√2)s2, or approximately 4.828427 × s2 (where s=side length)