| Interior Angle | = | 160° |
| Central Angle | = | 20° |
| Slice Area | ≈ | 0.171 r2 |
| Total Area | ≈ | 3.078 r2 |
| Circumference | ≈ | 6.251 r |
An octadecagon is a polygon with 18 sides, and the sum of its interior angles is 2880 degrees.
The values in the table above assume a regular polygon as in the picture. In a regular polygon, all angles and sides are equal, which means that a perfect circle can be drawn across all the vertices. The radius of this circle is called the circumradius, and is denoted r in the calculations above.
The more sides the regular polygon has, the more similar to a circle it becomes. As a result, the circumference gets closer and closer to 2 × π × r (≈ 6.283 r), and the area gets closer and closer to π × r2 (≈ 3.1415 r2).
Here are all the polygons with twenty or less sides:
| Number of Sides | Polygon Name |
|---|---|
| 3 | Triangle |
| 4 | Quadrilateral |
| 5 | Pentagon |
| 6 | Hexagon |
| 7 | Heptagon |
| 8 | Octagon |
| 9 | Nonagon |
| 10 | Decagon |
| 11 | Hendecagon |
| 12 | Dodecagon |
| 13 | Tridecagon |
| 14 | Tetradecagon |
| 15 | Pentadecagon |
| 16 | Hexadecagon |
| 17 | Heptadecagon |
| 18 | Octadecagon |
| 19 | Enneadecagon |
| 20 | Icosagon |