Systems and methods for rapidly and reliably determining an arch with of a patient's dental arch. A patient's dentition may be scanned and/or segmented. Arch width may be determined between points of intersection on the occlusal surface and a long axis of each tooth between one or more of: canine, first bicuspid, first primary molar, second bicuspid, second primary molar, and permanent first molar. Arch widths of different modified versions of the patient's dentition may be dynamically compared the patient's starting dentition, or to each other, and may be dynamically updated as the user modifies or switches between one or more 3D models of the patient's dentition.