Radii are related to circles.What do you mean by an "...octagon with a radius..."?Unless the octagon is regular it is impossible to to calculate the area of the octagon from one measurement alone.So assuming that the octagon is a regular octagon, what do you mean by radius? Do you mean:the radius of the circumcircle which passes through all verticesarea regular octagon = 8 × ½ × 8 × 8 × sin(360°/8) units² = 256 × sin(45) units²= 256 × 1/√2 units²= 128√2 units²≈ 181 units²the radius of the inscribed circle, which is the apothem of the octagonarea regular octagon = 8 × 8 × 8 × tan((360°/8)/2) units² ≈ 196 units²Something elseRe-ask your question explaining what you mean by "radius".