Thirty percent of India’s landmass is located in hilly regions, posing challenges for all-weather connectivity due to vulnerability to geo-hazards. Reinforcement geosynthetics such as geogrids and geocells hold promise in extending the service life of hill roads. However, the design of geogrid and geocell-reinforced pavements in still under development and requires further studies. Geocell-reinforced flexible pavement is designed based on the Mechanistic Empirical Approach outlined in IRC 37 (2018). This approach primarily relies on the ratio of the modulus of geosynthetics-confined base and subbase layers to the unconfined modulus of the same. This ratio, known as modulus improvement factor (MIF), plays a key role in optimizing the design of the geosynthetic-reinforced pavement. This study evaluates the MIF of locally available pavement material confined within geocells of a fixed depth but varying widths through numerical modeling in the three-dimensional finite element package PLAXIS 3D. The properties of the pavement material are considered based on values obtained from soil samples collected from the Jammu region of the Lesser Shivaliks. A single-cell honeycomb-shaped geocell pocket is simulated, with loading applied through a steel plate within the pocket. The study assesses the improvement in material modulus for different geocell pocket widths, analyzing stress–strain responses and load–deformation behavior. The study concluded that geocell reinforcement can enhance the modulus of the base/subbase layer, with the most significant improvement observed for geocell pockets with the smallest size. The modulus improvement factor (MIF) values obtained are within a similar range as reported in available experimental studies.