How to find the perimeter of a octagon

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.

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.