•Total acidity/pH value (TA/pH) is presented for the formulation designing of PCC;•The corrosion protection of PCC is improved with the decrease of TA/pH•Nucleation density and growth rate of PCC are strongly influenced by TA/pH In this work, with the aid of thermodynamic equilibrium calculations, a crucial contributor, i.e. total acidity/pH value (TA/pH), was proposed for the first time to guide the design of the chemical formulation of metal-phosphate-based conversion coatings having excellent corrosion resistance for Mg alloys. The present study and published works were compared to validate the proposed theory and correlation between the TA/pH and corrosion resistance of the resulting coatings. A significant contribution of this work was the elucidation of the kinetic role of the buffering nature of chemical bath in regulating the formation of corrosion resistant phosphate conversion coatings.