Multiply the length you measured by 8 or simply add up all eight sides. The answer should be the same and give you the circumference of the octagon shape. For example, if one of the sides of the octagon measures 3 inches, the circumference of the octagon would be 24 inches -- 3 x 8 = 24 or 3 + 3 + 3 + 3 + 3 + 3 +3 + 3 = 24.