Rare-earth (R) permanent magnets of R₂Fe₁₄B have technological importance due to their high energy products, and they have two R-sites (Wyckoff 4f and 4g, with four-fold multiplicity) that affect chemistry and valence. Designing magnetic behavior and stability via alloying is technologically relevant to reduce critical (expensive) R-content while retaining key properties;cerium, an abundant (cheap) R-element, offers this potential. We calculate magnetic propertiesand Ce site preference in ( R1−xCex) 2Fe14B [R = La,Nd] using density functional theory (DFT)methods—including a DFT + U scheme to treat localized 4f-electrons. Fe moments compare well with neutron data—almost unaffected by Hubbard U, and weakly affected by spin-orbit coupling.In La₂Fe₁₄B, Ce alloys for 0≤x≤1 and prefers smaller R(4f) sites, as observed, a trend we find unaffected by valence. Whereas, in Nd₂Fe₁₄B, Ce is predicted to have limited alloying ( x≤0.3 ) with a preference for larger R(4g) sites, resulting in weak partial ordering and segregation. The Curie temperatures versus x for (Nd,Ce) were predicted for a typical sample processing and verified experimentally.