r ( inradius ), the area is A = 8 \tan \frac{\pi}{8} r^2 = 8(\sqrt{2}-1)r^2 \simeq 3.3137085\,r^2. These last two coefficients bracket the value of pi , the area of the unit circle . The area can also be derived as follows: \,\!A=S^2-a^2, where S is the span of the octagon,