Packing efficiency = $\frac{volume of atoms in unit cell}{volume of unit cell}$ x 100. SC: (1 x 4/3 pi $r^3$) / (2r)^3 = pi/6 = 52.36%. BCC: (2 x 4/3 pi $r^3$) / (4r/sqrt(3))^3 = pisqrt$\frac{3}{8}$ = 68.02%. FCC: (4 x 4/3 pi $r^3$) / (2sqrt(2)*r)^3 = $\frac{pi}{3*sqrt(2}$) = 74.05%. HCP also has 74.05% — both are closest-packed. Void space = 100 - packing efficiency.