How to find the perimeter of a octagon

If you know the radius RR of the circle that inscribes a regular polygon having nn sides, the side of the regular polygon can be easily found with trigonometry.

The polygon is composed by nn identical triangles having a common vertex in the center of this circle, therefore this angle is 2π/n.

If it is a regular octagon, all sides are of equal length, so multiply the length of one side by 8 to get the perimeter. For an irregular octagon add together the length of each of the eight sides to get the perimeter.