An FCC unit cell has 8 corners, with each corner shared between eight unit cells. There are corner atoms present at alternate corners of the cube. Therefore, each corner atom is attributed to 1/8 to the particular unit cell. Thus, there is a total of 1 corner atom in an FCC unit cell.
There are 6 faces in a cube, and a cube has a total of 6 faces. Each face is shared by two adjacent unit cells, hence each face atom contributes 1/2 part to a unit cell. Therefore, the total number of face atoms belonging to a single unit cell is 6 * (1/2) = 3.
Hence there is 3 + 1 = 4 atoms in a FCC unit cell.
