Orthognathic surgery is used to treat dentofacial deformities and causes upper airway dimension changes. Surgical planning includes the study of the upper airway in CT images. The most common geometry-associated parameters used for the analysis are the total volume and the minimal cross-sectional area. However, these measurements do not provide information about the entire airway, potentially allowing irregularities and shape changes to go unnoticed. As a result, obtaining measurements of the cross-sectional areas along the entire upper airway is desirable. This information will prove useful for clinicians to compare changes after surgery and between patients. In this work, we introduce a simple technique for quantifying the cross-sectional area of the upper airway. We began by segmenting CT images of the upper airway. Later, we established its central axis and defined oblique slices perpendicular to this axis for measuring the cross-sectional area. Furthermore, we propose plotting these areas along the central axis, providing clinicians with an easily interpretable overview of the airway. The method was applied to nine patients, comparing pre-operative and post-operative images, clearly demonstrating changes in the cross-sectional areas after surgery. This method will allow for a better understanding of the effects and has the potential to improve the planning of orthognathic surgery. Further research will show how the movements of the jaw and maxilla influence the dimensions of the upper airway.