Number of voids per atom in hcp and ccp

The tetrahedral and octahedral voids created during hexagonal close packing occur in a fixed proportion to the number of particles packed. In both the hcp and the ccp structures, there are two tetrahedral voids per sphere and one octahedral void per sphere: if the number of particles is $N$, the number of tetrahedral voids is $2N$ and the number of octahedral voids is $N$. The number of octahedral voids is thus always half the number of tetrahedral voids.