it is direct formula. This can be seen in 3D fig. this is not theoretical. There is a simple way to calculate the number of Tetrahedral Voids in a lattice. Here if the number of spheres (unit cells) is said to be “n”, then the number of voids will be twice as many. So, the number of tetrahedral voids will be “2n”. The void is much smaller than the sphere, which means it has a smaller volume.