Introduction: The relationship between total mesiodistal widths of the maxillary and mandibular teeth is an important factor in orthodontic treatment planning. The purposes of this article are to report a mathematical tooth-size ratio specifically designed for patients needing the extraction of 4 first premolars and to compare the anterior "6" and overall "12" ratio values reported by Bolton with the calculated anterior "6" and overall "10" ratio values obtained from data in this study. Methods: This study was conducted in 3 phases. In the first 2 phases, we used the peer assessment rating and ideal cephalometric norms to select 53 ideal posttreatment models of patients who had had 4 premolars extracted. In the third phase, the mean overall "10" ratio and the mean anterior "6" ratio were calculated for the selected models. Bolton's mean overall 12" (91.3%) and anterior "6" ratios (77.2%) were compared statistically with calculations derived from this study by using 1-sample t test. Results: The mean overall ''10" ratio and the mean anterior "6" ratio were found to be 89.28 +/- 1.07% and 77.68 +/- 1.12%, respectively. Although the difference in anterior ratio was not significantly different from Bolton's anterior "6" ratio, there was a statistically significant difference between Bolton's study and our study in overall ratio. Conclusions: The mathematical tooth size overall ratio of 89.28% was determined for patients requiring the extraction of 4 first premolars and is recommended for use in diagnosis and treatment planning.