This paper develops a water retention model based on the soil mineralogy and microscopic features of the material behaviour. The model estimates the water content at a given total suction as the sum of the bentonite minerals interlayer water and the water in the interparticle pores. The paper proposes simple formulae to estimate the interlayer water content and interlayer void ratio based on mineralogical properties. Additionally, the model uses the mercury intrusion porosimetry measurements to approximate the water content in the larger pores. The validation of the proposed procedure relies on published data for MX-80 and GMZ bentonites and yields promising results. Even though the discussion and the validation are restricted to bentonite here, the procedure is generic in nature and can be modified easily for other geomaterials. The paper also offers insights into the links between soil microstructure and its macroscopic behaviour. The contribution provides a new tool for quick indirect estimation of the water retention behaviour of porous materials with known mineral composition and available microstructural data, e.g. based on the mercury intrusion porosimetry tests. Such tool is of high practical interest in design e.g. nuclear waste repositories where large material pool may be initially considered.