Objectives: The objective of this research was to determine the average concavity depth and width of first premolars in a group of Thais compared to the size of instruments used in periodontal therapy.

Materials and Methods: Among 260 extracted first premolars, 130 were maxillary and the other 130 mandibular. The width and depth of root concavities were measured at the coronal third and middle third with a surface-roughness tester. The width of the periodontal instruments was measured with Vernier calipers.

Results: Means and standard deviations of the concavity depth of the maxillary first premolar at the coronal third of mesial aspects, middle third of mesial aspects, coronal third of distal aspects, and middle third of distal aspects were 0.765±0.221 mm, 0.711±0.278 mm, 0.314±0.223 mm, and 0.504±0.250 mm, respectively. For the mandibular first premolar at the coronal third of mesial aspects, middle third of mesial aspects, coronal third of distal aspects, and middle third of distal aspects, values were 0.165±0.169 mm, 0.201±0.186 mm, 0.125±0.141 mm, and 0.139±0.132 mm, respectively. Means and standard deviations of the concavity width of the maxillary first premolar at the mesial and distal aspects were 0.836±0.607 mm and 1.874±0.976 mm, respectively, while for the mandibular first premolar at the mesial and distal aspects, values were 1.848±0.392 mm and 2.136±0.545 mm, respectively. The working-end widths of the instruments were 0.418–0.840 mm, and 18.46% of the mesial aspects of maxillary premolars were narrower than the smallest width of the instrument.

Conclusions: Based on this study, information on root concavities in first premolars in the Thai population will assist in better evaluation and treatment planning concerning the limited accessibility of instrumentation for use in root concavities, which can affect periodontal treatment outcomes.