Sum of interior angles of an n sided polygon = (2n - 4) * 90 degrees. Let us find the sum of the interior angles of a heptagon. Heptagon is a polygon with 7 sides. Sum of interior angles of a heptagon = (2 * 7 - 4) * 90 degrees. = 900 degrees. If heptagon is regular, then the value of an interior angle is $\frac{900}{7}$ degrees = 128.57 degrees.