what would the diameter be of a circle with an area of 15.89625

$$\begin{array}{rll} A&=&\pi r^2\\\\ 15.89625&=&\pi r^2\\\\ \frac{15.89625}{\pi}&=&r^2\\\\ \sqrt{\frac{15.89625}{\pi}}&=&r\\\\ \end{array}\\\\ d=2r=2\sqrt{\frac{15.89625}{\pi}}\quad units$$

$${\mathtt{2}}{\mathtt{\,\times\,}}{\sqrt{{\frac{{\mathtt{15.896\: \!25}}}{{\mathtt{\pi}}}}}} = {\mathtt{4.498\: \!859\: \!201\: \!286\: \!069}}$$

diameter is approx 4.5 units

http://web2.0calc.com/formulary/math/geometry/circle