Packing efficiency = (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(3)/8 = 68.02%. FCC: (4 x 4/3 pi r^3) / (2sqrt(2)r)^3 = pi/(3sqrt(2)) = 74.05%. HCP also has 74.05% — both are closest-packed. Void space = 100 - packing efficiency.