<p>The in situ stress state in rock masses, including its magnitudes and orientations, is critical for the design of a deep geological repository (DGR) for nuclear waste disposal. Reliable and accurate acquisition of in situ stress data is essential for this purpose. In this study, the diametrical core deformation analysis (DCDA) method is adapted to estimate in situ stress magnitudes for the first time. The effectiveness of the adapted method is verified through a comparison of both analytical and numerical solutions for major and minor horizontal stresses. Using a laser scan micrometer for measuring the diameters around the perimeter of selected core samples from a site recently chosen to be the future location for Canada’s DGR, it is found that their diameters all follow a sinusoidal trend, consistent with theoretical expectations. Subsequently, the adapted DCDA method is applied to calculate stress magnitudes. Results indicate that stress magnitudes generally increase with depth, reflecting the broader regional stress patterns in the Canadian Shield. At the studied site, stress magnitudes are generally within the variability range of all domains of the Canadian Shield stress database but exceed the best-fitted values based on regional trends at great depth. When compared to the Atomic Energy of Canada Limited-Underground Research Laboratory (AECL-URL) site, stress magnitudes at the future DGR site were generally lower though this difference diminishes with depth. Sensitivity analysis suggests that the stress magnitudes derived from the adapted DCDA method are largely insensitive to variations in Young’s modulus but highly responsive to changes in Poisson’s ratio. This study demonstrates that the adapted DCDA method provides a fast and cost-effective approximation of the in situ stress state, offering a valuable precursor to more costly and more time-consuming stress measurement methods such as overcoring or other complementary techniques.</p>