Packing fraction of a unit cell is defined as the fraction of the total volume of the unit cell ooccupied by the atom(s) 
$\begin{array}{l}\mathrm{P}.\mathrm{F}.=\frac{\text{ Volume of the atom }\left(\mathrm{s}\right)\text{ present in a unit cell }}{\text{ Volume of unitcell }}=\frac{\mathrm{Z}\times \frac{4}{3}{\mathrm{\pi r}}^3}{{\mathrm{a}}^3}\\ \text{ and }\mathrm{\%}\text{ of empty space }=100-\mathrm{P}.\mathrm{F}.\times 100\\ \text{ where }\mathrm{Z}=\text{ effective number of atoms in a cube; }\\ \mathrm{r}=\text{ radius of an atom; }\mathrm{a}=\text{ edge length of the cube. }\end{array}$