Purpose To define a method of quantifying axial proptosis in patients with Graves' orbitopathy (GO) and to validate a score that correlates with the orbital involvement and helps determine the degree of proptosis correction for elective orbital decompression. Design Retrospective, case series. Participants The study included 50 patients (group A) and 29 control subjects who underwent orbital computed tomography (CT). The method was then validated in another group of 21 patients with GO (group B). Methods The orbital area (OA) was measured manually on the central axial section of the CT scan at a level where the lens is visualized. The OA intersects the projection of the globe and delimitates the chord of an arch (globe chord [OC]). The area of the circular sector under the chord (CA) represents the portion of the globe within the orbit. Main Outcome Measures A CA-to-OA ratio was calculated to reduce the error due to variability of the measurements and to perform correlations with some of the clinical parameters of GO. Results Measurement error was low (<2%). We did not observe significant differences in the mean OA of patients with GO (783.6±12.1 mm2) and controls (758.5±20.4 mm2; P = not significant). The OC value in patients with GO was 130.2±11.5 mm2, significantly lower than in controls (281.8±9.7 mm2; P<0.0001). The CA-to-OA ratio also was lower in patients with GO than in controls (0.16±0.01 vs. 0.38±0.01; P<0.0001). A significant correlation was found in patients with GO between the CA-to-OA ratio and proptosis (P<0.001), lid fissure (P = 0.004), and intraocular pressure (P<0.001). In group B, the CA-to-OA ratio was 0.18±0.02, significantly different from that of controls (P<0.0001) and inversely correlated with proptosis (P<0.0001) and lid fissure (P<0.045). Conclusions By measuring the CA-to-OA ratio, we were able to quantify the degree of axial proptosis in patients with GO. The significant correlation of CA/OA with some orbital parameters confirms that this parameter also may be used as a measure of orbital involvement in GO. Financial Disclosure(s) The author(s) have no proprietary or commercial interest in any materials discussed in this article.