Cell of an HCP lattice is visualized as a top and bottom plane of 7 atoms, forming a regular hexagon around a central atom. In between these planes is a half-hexagon of 3 atoms.

There are two lattice parameters in HCP, a representing base and crepresenting height.

Coordination Number:

The number of atoms that surround a central atom in a solid is called coordination number.

Each atom in structure is positioned in the empty space formed by 3 adjacent atoms of the top layer and by 3 adjacent atoms in bottom layer and is surrounded by 6 neighbour atoms in the middle layer

All these 12 atoms are in contact with atom under consideration, hence the coordination number of the atoms in this structure is 12.

Effective Number of atoms per unit cell:

12 corner atoms shared by six unit cells each, two centre face atoms shared by two cells and three atoms fully contained by the unit cell.

Thus, $12\times\dfrac{1}{6}+\dfrac{2\times1}{2}+3=6$ atoms per unit cell for HCP. Hence, there are 6 number of atoms per unit cell.

Atomic Packing Factor :

The APF(atomic packing factor) is the ratio of the total sphere volume,  to the unit cell volume, .

For HCP, there are the equivalent of six spheres per unit cell, and thus-

(V = 6\left(\dfrac{4πR^3}{3}\right)=8πR^3)

Now, the unit cell volume is the product of the base area times the cell height, c.

The base area can be calculated as follows. The following figure shows an HCP unit cell and the basal plane. The base area is equal to six times the area of the equilateral triangle OAB.