...unit cell and volume of atoms within a unit cell for FCC...
Each face atom is shared by 2 unit cells and contributes 1/2 to the unit cell. Thus there are {eq}:(8\:\times\:\frac{1}{8})\:+\:(6\:\times\:\frac{1}{2})\:=\:1\:+\:3\:=4\:atoms\:per\:unit\:cell {/eq} in the case of FCC. ...