In the present work, the influence of the aspect ratio of the rectangular cavity (length > width) and the built-in rectangular blockage (length > width) on the local and global entropy generation (EG) characteristics due to double-diffusive convection and external magnetic field have been explored. Particularly, the effect of the pertinent parameters, such as the aspect ratio of the cavity (Formula presented.) and blockage (Formula presented.) Lewis number (Formula presented.) Rayleigh number (Formula presented.) Hartmann number (Formula presented.) and buoyancy ratios (Formula presented.) on the EG characteristics have been explored. The working fluid is considered to be a liquid metal-sodium–potassium alloy (Pr = 0.054). Lattice Boltzmann method numerical scheme is employed to solve the double-diffusive natural convection phenomenon. The increase in Ra induces an enhancement in the localized EG terms, whereas the augmentation in Ha diminishes the local total EG (S Total) for a given Ra. The dominance of thermal and species transport irreversibility (Formula presented.) in the cavity was noticed for (Formula presented.) Conversely, for AR = 4, the significance of flow friction irreversibility within the cavity was observed ((Formula presented.)). The results of the work are also summarized in terms of simpler empirical correlations of (Formula presented.) and ST, which might be useful for possible engineering design purposes. © 2022 Taylor & Francis Group, LLC.