Background: The overall changes of ocular motility in Graves' orbitopathy (GO) are not easily quantifiable with the methods currently available, especially in clinical studies. The aim of the present study was to calculate parameters that quantify the changes of ocular motility in GO in relation to the Gorman score for diplopia. Methods: We studied 100 GO patients (Group 1) and 100 controls (Group 2). We also included 30 patients treated with intravenous methylprednisolone (iv-MP), assessed at baseline and after 12 and 24 weeks (Group 3), and 66 patients submitted to squint surgery, assessed at baseline and after 12 weeks (Group 4). Ocular ductions were measured in four gaze directions by a perimeter arc and were used to calculate a total motility score (TMS) as the sum of ductions in each direction; a biocular TMS (b-TMS) as the sum of the TMS of two eyes; and an asymmetry ratio (AR) as the sum of the differences of the corresponding ductions between the two fellow eyes divided by the mean difference found in controls. Quality of life was accessed by a specific questionnaire (Graves' orbitopathy quality of life [GO-QoL] questionnaire). Results: TMS and b-TMS were lower, while AR was higher, in Group 1 compared with controls (p < 0.001). In Group 1, TMS and b-TMS were inversely correlated with the Gorman score (p < 0.001) and AR was higher in patients with constant diplopia compared with the others (p < 0.001). In Group 3, TMS and b-TMS increased after treatment in responders to iv-MP (p < 0.001). In Group 4, TMS and b-TMS improved in all patients after surgery (p < 0.01), while AR and GO-QoL score improved only in those without residual constant diplopia (p < 0.001). Conclusion: We describe a quantitative method to assess eye motility dysfunction in any stage of GO to be used as an outcome measure in clinical studies.