The purpose of this study was to assess the alveolar defect volume in unilateral cleft lip and palate (UCLP) subjects using computed tomography (CT) and a free software program to evaluate the intra-and interrater measurements, and to compare the cleft volume between age and affected side. The sample of this retrospective study consisted of 20 UCLP individuals, 12 boys and 8 girls, mean age 10.3 ± 2.4 years at the beginning of orthodontic treatment. All subjects required alveolar bone grafting. CT scans of the cleft area were obtained prior to secondary bone grafting, and were analyzed using Image J. software program. The cleft volume was calculated based on axial crosssectional CT images by two raters (orthodontist and radiologist) and by the same rater (orthodontist) at two different moments. Linear mixed model, Bland-Altman, Pearson's and intraclass correlation coefficient (ICC) were used. The mean cleft volume was 7.53 ± 1.55 mm³. The intra-and inter-rater measurements were reproducible (ICC = 0.976 and 0.963, respectively) with no significant difference between them. There were no statistically significant differences in the cleft volume related to age or cleft location. The assessment of cleft volume in UCLP using CT images and a free software program was a reproducible method. There was no significant relation between alveolar defect volume and age or cleft location.