Copyright © 2020 Wolters Kluwer Health, Inc. All rights reserved.MINI: The authors developed a mathematical model to the sagittal vertical axis (SVA) change in ankylosing spondylitis whom PSO is planned. The mathematical model was developed using trigonometric equations. No significant difference exists between postop SVA change amount and SVA calculated. The mathematical model is reliable in restoring the global sagittal balance. Retrospective study. This study aims to develop a mathematical model to help precalculate the sagittal vertical axis (SVA) change in patients with ankylosing spondylitis (AS) with rigid kyphotic deformity for whom pedicle subtraction osteotomy (PSO) is planned. SVA is an important metric parameter used to evaluate the global sagittal balance. Previous studies have investigated angular changes in pelvic parameters using PSO; however, no mathematical model is available to calculate SVA change as a metric in these studies. Twenty-one patients who met the inclusion criteria were included in the study. The mathematical model was developed using basic trigonometric equations. Measurements for SVA, lumbar lordosis (LL), pelvic tilt (PT), sacral slope (SS), pelvic incidence (PI), and the mathematical model were performed in the preop and early postop period. The amount of SVA change in the poststop period was calculated in the mathematical model. The mean age was 33.81 ± 6.01 years. No statistical difference was observed between MATLAB and the angles used in the mathematical modeling (P > 0.05). No significant difference exists between postop SVA change amount and SVA calculated via mathematical modeling (P > 0.05). A statistically significant difference was observed between preop and postop measurements of LL, SVA, PT, and SS variables (P < 0.001). No statistically significant difference existed between PI (P > 0.05). This novel mathematical model is reliable in restoring the global sagittal balance of the patients with AS scheduled for PSO and prevent the osteotomy complications. Level of Evidence: 3.