The face diagonal is part of a right triangle that includes two unit cell edges.

f, the length of the face diagonal can be calculated from a, the length of the cell edge using the Pythagorean Theorem.

The face diagonal is four times the atomic radius (f = 4R) so

For the FCC metal Pt: a= 392 pm
R = 139 pm

Click the link on the right to view the pythagorean theorem in action. Click the underlined text above to view a definition.