In a face-centered cubic (fcc) lattice, there are four atoms per unit cell. The octahedral voids in an fcc lattice are located at the center of the unit cell and at the center of each edge.

The total number of octahedral voids in FCC unit cell are four.

Octahedral voids in FCC = Edge centres + Body centre.

The contribution of edge centre = $\frac{1}{4}$

$\therefore$ Fraction of one edge centred octahedral void in one unit cell of FCC = $\frac{1}{4}$