In fcc, interstitial sites will be octahedral voids and tetrahedral voids
For tetrahedral voids
r1/r2 = 0.225
For octahedral voids
r1/r2 = 0.414
where r1 → radius of atom in interstitial sites
r2 → radius ofatom arranged in fcc
so 4r2 = √2 a,
For maximum diameter of atom in interstitial site, octahedral voids will be considered