A study has been undertaken of the variation of the magnetic loss factor with the evolution of fired relative density and microstructure of ferrites made from a Zn-Ni-Cu ferrite powder, and from a mixture of a Zn-Ni and a Cu ferrite powder. An equation is proposed that enables relating the magnetic loss factor to fired relative density, average grain size, and grain size distribution width. This equation is valid for the ferrites studied, which had an average grain size below 10 mum and a grain size distribution width below 30 mum.