RESUMEN
The severity of dental caries which pulp exposes are mostly extracted. Endodntic treatment is the best choice are limited at apical constriction (cemento-dentinal junction) which average distrance 0.5 to 1 milimeter from root apex. The purpose of this study is the measuring of tooth length from the heighest point of crown to root apex. One thousand and two hundred endodontic patients, more than nine years old, has been studied at the dental department, Siriraj Hospital, Mahidol University. The treated tooth has been measured tooth length before and after instrumentation by endodontic file. The result are should that male to female patient are 1:1.86 and average tooth length as followed; upper central incisor, lower central incisor, upper lateral incisor, lower lateral incisor, upper canine, lower canine were 21.7+1.87, 18.7+1.96, 20.4+1.94, 19.8+1.46, 23+1.78 and 21.2+1.65 millimeters respectively. The average length of upper first bicuspid, lower first bicuspid, upper second bicuspid, lower second bicuspid, upper first molar, lower first molar, upper second molar, lower second molar, upper third molar and lower third molar were 18.9+1.45, 19.7+1.42, 18.9+1.47, 19.4+1.68, 18.3+1.58, 18.8+1.32, 18.2+1.74, 18.2+1. 4, 17+0.81 and 16.7+1.68 millimeters respectively. The average tooth length is guided to enlarge and filling root canal in the process of endodontic treatment.