Assuming one atom per lattice point, in a primitive cubic lattice with cube side length "a", the sphere radius would be and the atomic packing factor turns out to be about 0.524 (which is quite low). Similarly, in a "bcc" lattice, the atomic packing factor is 0.680, and in "fcc" it is 0.740. The "fcc" value is the highest theoretically possible value for any lattice, although there are other lattices which also achieve the same value, such as hexagonal close packed ("hcp") and one version of tetrahedral "bcc".