An octagon is a polygon that has eight sides. The internal angle at each vertex of a regular octagon is 135°. The area of a regular octagon of side length a is given by

<math>A = 2a^2 \cot \frac{\pi}{8} = 2(1+\sqrt{2})a^2 \simeq 4.82843 a^2.<math>

		Missing image Concave_octagon.png Concave Octagon	Missing image Complex_octagon.png Complex Octagon
regular	irregular	concave	complex

Worldwide, stop signs are regular octagons.

See also

• octagonal number
• decagon
• dodecagon
• enneagon
• geometry
• heptagon
• hendecagon
• hexagon
• pentagon
• quadrilateral
• triangle

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In architecture, an octagon house is a house built in an octagonal shape. Monticello is the most famous example.

In mixed martial arts, The Octagon is the name of the octagonal ring used in the Ultimate Fighting Championship, and by metonymy a term for the UFC itself as well.

The Octagon is the name of the city centres of Bristol, England, and Dunedin, New Zealand.

de:Achteck sv:Oktagon