Iron occurs in two cubic Bravais lattices. The ratio of their densities is 0.92. The Bravais lattices, respectively, are 

${\rho }_1=\frac{N_1\left(M/N_{\mathrm{A}}\right)}{a_1^3}$ and ${\rho }_2=\frac{N_2\left(M/N_{\mathrm{A}}\right)}{a_2^3}$ Hence, $\frac{{\rho }_1}{{\rho }_2}=\frac{N_1}{N_2}{\left(\frac{a_2}{a_1}\right)}^3$ 

For primitive and body-centred cubic system $\frac{{\rho }_{\mathrm{P}}}{{\rho }_{\mathrm{l}}}=\left(\frac{1}{2}\right){\left[\frac{\left(4\sqrt{3}\right)r}{2r}\right]}^3=0.77$

For primitive and face-centred $\frac{{\rho }_{\mathrm{P}}}{{\rho }_{\mathrm{F}}}=\left(\frac{1}{4}\right){\left[\frac{2\sqrt{2}r}{2r}\right]}^3=0.71$

For body-centred and face-centred cubic systems $\frac{{\rho }_{\mathrm{I}}}{{\rho }_{\mathrm{E}}}=\left(\frac{2}{4}\right){\left[\frac{2\sqrt{2}r}{2r}\right]}^3=0.92$