Interest in microbially induced calcite precipitation (MICP) has grown due to the demand for sustainable and energy-efficient soil improvement methods. This study explored the potential of using Sporosarcina pasteurii to enhance the engineering properties of sandy soils with varying grain sizes and relative densities. Calcium carbonate precipitation induced by bacterial activity was assessed under different bacterial concentrations (10⁷, 10⁸, and 10⁹ cells/mL) and temperatures (16 °C, 30 °C, and 45 °C). The improvements were evaluated using unconfined compressive strength (UCS) tests and microstructural analyses using SEM, EDS, and XRD techniques. The results indicated that MICP significantly increased soil strength, with the highest UCS values observed for medium and coarse sands under optimal conditions. Fine sand exhibited limited improvement owing to lower permeability, which hindered bacterial distribution. SEM and XRD analyses confirmed the presence of calcium carbonate polymorphs, such as calcite and vaterite, enhancing intergranular bonding. The optimal conditions for bacterial activity and calcium carbonate precipitation were a concentration of 10⁹ cells/mL and temperatures of 30 °C for fine sand and 45 °C for coarse sand. This research underscores the potential of MICP as a sustainable soil stabilisation technique while highlighting challenges in bacterial distribution and bonding across different sand types.