The formula for the sum of the interior angles of a polygon is
(n-2) * 180 where n is the number of sides
An octagon has 8 sides so n = 8
(8-2) * 180
6 * 180
1080
Since it is regular, that means all the angles are the same
We can divide by the number of angles to find the measure of each angle
1080/8 =135
Each angle is 135