This study investigates the influence of infill geometry and density on the effective cross-sectional area of PLA components manufactured using fused deposition modeling (FDM). Specimens with triangular, square, hexagonal, and circular internal infill patterns were fabricated and evaluated under tensile loading according to ISO 527–2:2012. Each infill type was tested at various orientations (0°, 45°, and 90°) and two levels of density: standard (R00) and increased (R01). The total, wall, and effective cross-sectional areas were obtained using digital image analysis supported by integrated software on a Testometric X350–5 tensile testing machine. The results indicate that infill geometry significantly affects material distribution within the cross-section. Among reference samples, the hexagonal infill at 90° exhibited the highest effective area (14.40 mm2), while the square infill showed the lowest (8.80 mm2). After increasing infill density (R01), the effective area of hexagonal patterns reached 23.19 mm2, representing an 83% increase. The experimental data further demonstrate a direct correlation between the reduction in effective cross-sectional area and the loss of tensile strength. Infill configurations with lower structural continuity and density consistently exhibited inferior mechanical performance. The circular infill demonstrated the most isotropic behavior across orientations. The study proposes the effective cross-sectional area as a predictive geometric parameter for estimating material strength and simulating load-bearing behavior. These insights contribute to the structural optimization of 3D-printed components and inform performance-driven design strategies in additive manufacturing.