Packing efficiency of a simple cubic (sc) structure is:

Sc has 1 sphere per unit cell of edge $a = 2r$. Volume of sphere = $(4/3)\pi r^3$; volume of cube = $a^3 = 8r^3$. Packing efficiency = $((4/3)\pi r^3)/8r^3 = \pi/6 \approx 52.4\%$. Least efficient — Po is the only element with sc structure.