<p>The prediction of the hydraulic conductivity of fine-grained soils has garnered significant attention. Although the Kozeny–Carman (KC) equation is currently the most widely used model, it struggles to account for the effects of adsorption, electrostatic interactions, and other factors related to chemical oxidants. This study aims to enhance the applicability of the KC equation for predicting hydraulic conductivity in terms of particle size of fine-grained soils. A modified version of the KC model is proposed by introducing the concept of equivalent particle size, which incorporates the effect of water adsorbed effect. This model is evaluated by 1286 experimental data spanning over 12 orders of magnitude from this study and existing literature. Building on numerical simulations and experimental data, this study introduces two characteristic equivalent particle sizes: the critical particle size and the threshold particle size, and explicitly defines their values. The critical particle size (1.0 × 10^–3&#xa0;mm) serves as a criterion for identifying the influence of adsorption on seepage and as a boundary for the applicability of the classical KC model. It is particularly relevant for soils with particle sizes larger than the critical value. The threshold particle size (1.0 × 10^–5&#xa0;mm), on the other hand, represents a limiting particle size, above which water can effectively flow through the soil and the hydraulic conductivity can be tested. However, both of these two equivalent characteristic particle sizes are influenced by the thickness of the adsorbed water film, and further research is needed to determine more accurate and rational values for them.</p> Graphical abstract <p></p>