Alternatively, if we know the radii of the ions in a CaF₂ -like cell, we can estimate the cell dimension. We estimate the edge length of the unit cell of CaF₂ from published values of the radii:
The radius of Ca²⁺ is 100 pm.
The radius of F ^- is 131 pm.

The body diagonal is 462 pm long.

The length of the edge of the small cube is 267 pm, thus the unit cell length is 534 pm. The distance between the center of the fluoride ion and the center of the calcium ion is 1/2 the length of the body diagonal of the small cube or 1/4 the length of the body diagonal of the unit cell.

Click the links on the right to illustrate these concepts.