Tooth numbering systems: the Universal 1 to 32 and A to T, the FDI two-digit ISO 3950, and Palmer, with the conversion both ways

What it says#

The ADA's value set states the position plainly: the American Dental Association accepted the Universal/National Tooth Designation System and the ISO/ANSI/ADA Specification No. 3950 for Designation System for Teeth and Areas of the Oral Cavity as the human tooth and oral cavity enumeration schemas in 1994, and the Universal system is the one most commonly used by dental professionals in America.

Universal permanent teeth are numbered 1 to 32, starting with the third molar (1) on the right side of the upper arch, following around the arch to the third molar (16) on the left side, descending to the lower third molar (17) on the left side, and following that arch to the lower right third molar (32). Primary teeth are lettered A to T, starting at the posterior of the upper right quadrant, where the right second molar is A, following around to J at the upper left second molar; K is the mandibular left second molar and the lettering continues to T at the mandibular right second molar. Supernumerary permanent teeth take the numbers 51 through 82 on the same path, so supernumerary 51 is adjacent to tooth 1 and supernumerary 82 is adjacent to tooth 32. A supernumerary primary tooth takes the letter of its neighbor plus S, so AS is adjacent to A and TS is adjacent to T.

The FDI two-digit system, which is ISO 3950, uses one digit for the quadrant and one for the tooth. The ADA's value set gives the mapping tooth by tooth, and reading it out is the fastest way to learn the pattern. Universal 1 is ISO 18, 2 is 17, 3 is 16, 4 is 15, 5 is 14, 6 is 13, 7 is 12, 8 is 11; then Universal 9 is 21 and the numbers climb to Universal 16, which is 28. Universal 17 is 38 and the numbers fall to Universal 24, which is 31; Universal 25 is 41 and they climb again to Universal 32, which is 48. The primary teeth follow the same logic one quadrant series higher: A is 55, B 54, C 53, D 52, E 51, F 61, G 62, H 63, I 64, J 65, K 75, L 74, M 73, N 72, O 71, P 81, Q 82, R 83, S 84, T 85. Read that list and the rule falls out: the first digit is the quadrant, 1 upper right, 2 upper left, 3 lower left, 4 lower right for permanent teeth and 5, 6, 7, 8 in the same order for primary teeth, and the second digit counts outward from the midline, 1 for the central incisor to 8 for the third molar, or 1 to 5 in the primary dentition.

Palmer notation is the oldest of the three. The Saudi Dental Journal survey of tooth numbering systems traces it to Adolf Zsigmondy of Vienna in 1861, who numbered the permanent teeth 1 to 8 from the central incisor out to the third molar and used roman numerals I to V, later modified to A to E, for the primary teeth, placing a grid symbol around the number or letter to show the quadrant. Corydon Palmer of Ohio described a similar method in 1870 without knowing of Zsigmondy's work, which is why English-speaking countries call it the Palmer system. The same paper records that the two-digit system was first described by Jochen Viohl of Berlin in 1966, that the FDI proposed at its 1970 annual meeting that it be used worldwide because it is simple to teach, easy to pronounce, readily communicable in print and easy to translate into computer output, and that by 1989 it had been adopted by ISO/TC 106, the British Standards Institution, the World Health Organization, Interpol and the International Association for Dental Research.

For claims, the ADA's completion instructions fix the vocabulary. Item 26, Tooth System, takes JP to indicate the ADA's Universal/National Tooth Designation System, 1 to 32 for permanent and A to T for primary. Item 27 takes the tooth number or letter, with ranges written either with a hyphen (1-4; 7-10; 22-27) or with commas separating numbers and ranges (1, 2, 4, 7-10). Item 25, Area of Oral Cavity, uses a separate two-digit code list: 00 entire oral cavity, 01 maxillary arch, 02 mandibular arch, 10 upper right quadrant, 20 upper left quadrant, 30 lower left quadrant, 40 lower right quadrant. Item 28 takes the surface letters B buccal, D distal, F facial or labial, I incisal, L lingual, M mesial, O occlusal, run together with no spaces, as in MOD. And there is one note worth reading twice: numbers or letters reported are based on tooth morphology, not anatomic position, so a tooth that has migrated into an edentulous space keeps its own number, and an implant prosthesis is reported as the morphological equivalent of the missing tooth.

Who it applies to#

Everyone who writes a tooth on a chart, a referral or a claim, which is the whole clinical team plus the front desk. It matters most in three situations: an associate, hygienist or assistant who trained outside the United States and learned FDI or Palmer first; a US dentist reading a referral, a radiograph report or a records transfer from another country; and a specialist receiving a referral where a transposition of the two systems would send a bur to the wrong tooth.

It does not change what goes on an ADA claim form, which takes Universal only. The value set says the ISO designations are included to assist with mapping and should not be used on the ADA claim form. Quadrants, arches and the whole mouth are not tooth numbers; they are areas of the oral cavity with their own codes.

What changed#

Nothing in the systems themselves. The ADA accepted both schemas in 1994, the claim form instructions carry a completion date of February 11, 2022, and the value set carries a July 2024 version date. What has changed is the amount of cross-border traffic in dental records, and what has not changed is that the two systems overlap dangerously: 16, 17, 18, 21 to 28, 31 to 32 and 41 to 48 are all legitimate readings in both notations, and only context tells you which one you are looking at. In Universal, 16 is the upper left third molar; in ISO 3950, 16 is the upper right first molar.

Where the source is#

The link under the title opens the ADA's Universal Tooth Designation System value set, a seven-page PDF dated July 2024 and copyright 2024 American Dental Association, which was downloaded and read in full for this entry. Every row of that document names both designations, for example "Permanent maxillary right first molar tooth; Universal designation 3; ISO designation 16", and the primary rows do the same from A to T; the mapping in the figure was copied from those rows, not reconstructed. The companion ADA Dental Claim Form Completion Instructions, version 2019 with a completion instructions date of February 11, 2022, supplied the JP qualifier, the supernumerary charts, the area of oral cavity code list and the surface letters. The Palmer history and the FDI adoption record come from Al-Johany's open-access survey in the Saudi Dental Journal, listed in the downloads, which is published under a Creative Commons license. ISO 3950 itself is a paid standard and was not purchased for this entry; the ISO designations reported here are the ones the ADA prints.