Octadecagon

A octadecagon or 18-gon is a shape with 18 sides and 18 corners.

A octadecagon

Regular octadecagon

All sides of a regular octadecagon are the same length. Each corner is 160°. All corners added together equal 2880°.

The amount of space a regular octadecagon takes up is

The area of a regular octadecagon of side length a is given by

A={\frac {18}{4}}a^{2}\cot {\frac {\pi }{18}}\simeq 25,5208a^{2}

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Polygons
Special cases, including regular polygons with their own names, in parentheses List of polygons by number of sides
1–10 sides	Henagon Digon Triangle Quadrilateral Pentagon Hexagon Heptagon Octagon Nonagon Decagon
11–20 sides	Hendecagon Dodecagon Tridecagon Tetradecagon Pentadecagon Hexadecagon Heptadecagon Octadecagon Enneadecagon Icosagon
21–30 sides	Icosihenagon (21) Icosidigon (22) Icosikaitrigon (23) Icosikaitetragon (24) Icosipentagon (25) Icosihexagon (26) Icosikaiheptagon (27) Icosioctagon (28) Icosikaienneagon (29) Triacontagon (30)
31–50 sides	Triacontadigon (32) Triacontatetragon (34) Tetracontagon (40) Tetracontadigon (42) Tetracontaoctagon (48) Pentacontagon (50)
51–100 sides (selected)	Hexacontagon (60) Hexacontatetragon (64) Heptacontagon (70) Octacontagon (80) Enneacontagon (90) Enneacontahexagon (96) Hectogon (100)
>100 sides	120-gon 257-gon 360-gon Chiliagon (1,000) Myriagon (10,000) 65537-gon Apeirogon (∞)
Star polygons (5–12 sides)	Pentagram Hexagram Heptagram Octagram Enneagram Decagram Hendecagram Dodecagram
Triangles	Isosceles triangle Equilateral triangle Right triangle
Quadrilaterals	Square Rectangle Rhombus Parallelogram Trapezoid Kite Antiparallelogram