+118691 $$\begin{array}{rll}
A&=&\pi r^2\\\\
15.89625&=&\pi r^2\\\\
\frac{15.89625}{\pi}&=&r^2\\\\
\sqrt{\frac{15.89625}{\pi}}&=&r\\\\
\end{array}\\\\
d=2r=2\sqrt{\frac{15.89625}{\pi}}\quad units$$
$${\mathtt{2}}{\mathtt{\,\times\,}}{\sqrt{{\frac{{\mathtt{15.896\: \!25}}}{{\mathtt{\pi}}}}}} = {\mathtt{4.498\: \!859\: \!201\: \!286\: \!069}}$$
diameter is approx 4.5 units
+118691 $$\begin{array}{rll}
A&=&\pi r^2\\\\
15.89625&=&\pi r^2\\\\
\frac{15.89625}{\pi}&=&r^2\\\\
\sqrt{\frac{15.89625}{\pi}}&=&r\\\\
\end{array}\\\\
d=2r=2\sqrt{\frac{15.89625}{\pi}}\quad units$$
$${\mathtt{2}}{\mathtt{\,\times\,}}{\sqrt{{\frac{{\mathtt{15.896\: \!25}}}{{\mathtt{\pi}}}}}} = {\mathtt{4.498\: \!859\: \!201\: \!286\: \!069}}$$
diameter is approx 4.5 units
