<p>Evaluating the mechanical behaviours of intercalated shale-sandstone, generally associated with coal mines, is a challenging task as the proportions of the shale and sandstone units vary in sedimentary depositional environments. This study proposed a new feasible method to estimate the tensile strength of such rocks during on-site field investigations. This method, termed “Gridding”, was used to assess the gridded shale content (%) of intercalated shale-sandstone macroscopically, which was substantiated by microscopical data. In this regard, core samples from the Barakar Formation (central-eastern India) and Raniganj Formation (eastern India) were utilised. Subsequently, the relation between the gridded shale content (%) and tensile strength was scrutinised for a total of 112 specimens from both formations, and the results that were obtained were compared and validated. It was observed that the tensile strength of the intercalated shale-sandstone increased until the shale content (%) reached a threshold of 55 to 65%; thenceforth, the strength declined. The plausible reason for this observed pattern was deduced with the help of microstructural studies (using a petrographic microscope and a scanning electron microscope). A conspicuous influence of rock grain size and shale-induced cementation on failure modes was recognised. Subsequent three-dimensional (3D) finite element model analysis also endorsed this hypothesis. Finally, a classification for intercalated shale-sandstone based on these findings was proposed, which could be used for on-site field investigation for estimating the tensile strength with regard to the quantified gridded shale content (%).</p>