The sum of the interior angles \( S \) of a convex polygon can be calculated using the formula:
\[ S = (n - 2) \times 180 \]
where \( n \) is the number of sides of the polygon. For a polygon with 9 sides:
\[ S = (9 - 2) \times 180 \] \[ S = 7 \times 180 \] \[ S = 1260 \]
Therefore, the sum of the interior angles of a convex polygon with 9 sides is 1260 degrees.