We present a computational study of the compound Y Co 1 - x - y Fe x Cu y 5 for 0 ⩽ x , y ⩽ 0.2 . This compound was chosen as a prototype for investigating the cell boundary phase believed to play a key role in establishing the high coercivity of commercial Sm-Co 2:17 magnets. Using density-functional theory, we have calculated the magnetization and magnetocrystalline anisotropy at zero temperature for a range of compositions, modeling the doped compounds within the coherent potential approximation. We have also performed finite temperature calculations for YCo 5 , Y Co 0.838 Cu 0.162 5 and Y Co 0.838 Fe 0.081 Cu 0.081 5 within the disordered local moment picture. Our calculations find that substituting Co with small amounts of either Fe or Cu boosts the magnetocrystalline anisotropy K, but the change in K depends strongly on the location of the dopants. Furthermore, the calculations do not show a particularly large difference between the magnetic properties of Cu-rich Y Co 0.838 Cu 0.162 5 and equal Fe-Cu Y Co 0.838 Fe 0.081 Cu 0.081 5 , despite these two compositions showing different coercivity behavior when found in the cell boundary phase of 2:17 magnets. Our study lays the groundwork for studying the rare earth contribution to the anisotropy of Sm Co 1 - x - y Fe x Cu y 5 , and also shows how a small amount of transition metal substitution can boost the anisotropy field of YCo 5 .