Relevant information can be drawn from the airway geometry, including assessments of its health and parameters used in the study of airflow. The cross-sectional area of the tracheal lumen is one of the broadly useful geometrical properties of the airways. Therefore, the present work describes a methodology for obtaining the cross-sectional area of the tracheal lumen as a function of the axial coordinate. Cross-sectional area should be understood as the area calculated based on a cutting-plane orthogonal to the centerline of a geometry. The airway geometries were obtained through CT scan image segmentation. The methodology was validated with the use of two virtual phantoms of known dimensions, achieving a mean cross-sectional area error of \(<1\) %. Additionally, the tracheae of two subjects were used to exemplify the applicability of our algorithm: a healthy rabbit and a patient whose airways had been deformed by tumors. Since the cross-sectional area is calculated based on the centerline of the geometry, the area obtained by the proposed method is independent of the subject’s position in the CT scanner.