| Interior angle | 140° | Like any regular polygon, to find the interior angle we use the formula (180n–360)/n . For a nonagon, n=9. See Interior Angles of a Polygon |
| Exterior Angle | 40° | To find the exterior angle of a regular decagon, we use the fact that the exterior angle forms a linear pair with the interior angle, so in general it is given by the formula 180-interior angle. See Exterior Angles of a Polygon |
| Area | 6.182s2 approx | Where S is the length of a side. To find the exact area of a decagon or any polygon, using various methods, see Area of a Regular Polygon and Area of an Irregular Polygon |
| Number of diagonals | 27 | The number of distinct diagonals possible from all vertices. (In general ½n(n–3) ). In the figure above, click on "show diagonals" to see them. See Diagonals of a Polygon |
| Number of triangles | 7 | The number of triangles created by drawing the diagonals from a given vertex. (In general n–2). In the figure above, click on "show triangles" to see them. See Triangles of a Polygon |
| Sum of interior angles | 1260° | In general 180(n–2) degrees . See Interior Angles of a Polygon |