So, the sum of the interior angles of a nonagon is 1260 degrees. Regular Nonagons: All sides are the same length (congruent) and all interior angles are the same size (congruent). To find the measure of the angles, we know that the sum of all the angles is 1260 degrees (from above)... And there are nine angles... So, the measure of the angle of a regular nonagon is 140 degrees.