Passive stabilization is a key feature in tokamak design since it indicates the efficiency of the metallic structures to 'oppose' plasma displacements. As far as plasma vertical displacement modes are concerned, usually their passive stabilization is characterized in terms of two main indices, namely the instability growth time and the stability margin. In this study—after recalling the governing equations—we extend the definition of the stability margin given in the literature (Lazarus E. et al1990 Nucl. Fusion30 111, Albanese R. et al 1990 IEEE Trans. Magn.26, Kameari A. et al 1985 Nucl. Eng. Des./Fusion 365–73) for the rigid body displacement model to the non-rigid plasma model. Numerical examples are also given for the reduced task objectives/reduced cost ITER design.