For the sake of completeness:
Aread of sqaure = $d$
then, side of sqaure = $\sqrt(d)$
length of diagonal (using $a^2 + b^2 = c^2$), = $\sqrt(2d)$
as this is the diameter of the circle, radius of the circlet = $\sqrt(\frac{d}{2})$
Area of circle, using $\pi * r^2$ = $\pi * \frac{d}{2}$