Based on the physical picture of free-volume creation and diffusion via shear transformations (STs), we develop a model for predicting the shear band dilatation in amorphous alloys. The model reveals that the dilatation in shear bands with fixed thickness increases with decreasing activation volume of STs that determines the diffusion distance of free-volume. Mature (or thicker) shear bands possess higher dilatation than recently evolved, thinner shear bands. The predicted dilatations agree well with the values of measurements and simulations, clarifying the long-standing controversy. The underlying physics is attributed to the local topological instability via STs that leads to shear banding. (C) 2016 Acta Materialia Inc Published by Elsevier Ltd. All rights reserved.