Background and Objective: Dental study casts play a vital role in the diagnosis and treatment planning of various orthodontic cases. This study was carried out to compare the tooth widths, arch widths, and arch lengths in Class-I normal dentition to those in Class-I and Class-II crowded dentition in an effort to improve treatment planning and to eventually reduce treatment duration. Methods: Total 170 patients, 12 to 40 years of age with a complete set of permanent teeth till 1st molars; who presented to the Orthodontics Department at Armed Forces Institute of Dentistry (A.F.I.D), Rawalpindi from Sep 2019 to Feb 2020, were included in the study. Non-probability purposive method of sampling was used. The dental casts obtained were used to measure tooth widths, arch widths, and arch lengths. Subjects were classified into Class-I normal and Class-I and Class-II crowded occlusion and comparison of the sum of tooth widths, arch widths, and arch length discrepancies were determined among the three occlusion groups. Data was analyzed in SPSS version 21 and independent samples t-test was used to differentiate the variables of interest. Results: Out of 170 subjects, 73 (42.9%) subjects had Class-I normal occlusion while 97 (57%) had Class-I and Class-II crowded occlusions. No statistical difference was found between the occlusal groups with regard to the sum of tooth widths, inter-canine widths, inter-first premolar widths, inter-second premolar widths and inter-molar widths. However, a remarkable difference was observed between the occlusal groups with respect to arch perimeters and arch length discrepancies (p = 0.000 and 0.000 respectively). Conclusions: Results of the current study indicate that crowding of teeth occurs as a consequence of decreased arch perimeters which may lead to increased arch length discrepancies. However, no prominent difference was noticed in the sum of tooth widths and arch widths among different occlusal groups. doi: https://doi.org/10.12669/pjms.37.2.3240 How to cite this:Shafique HZ, Zaheer R, Jan A, Fazal A. Comparison of Tooth Widths, Arch Widths and Arch Lengths in Class-I Normal Dentition to Class-I and II Crowded Dentitions. Pak J Med Sci. 2021;37(2):345-350. doi: https://doi.org/10.12669/pjms.37.2.3240 This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.