When does tooth K come in and fall out?
The lower baby second molars usually come in at 23–31 months and fall out at 10–12 years (ADA). The AAPD chart gives 20–30 months to come in and 11–13 years to fall out.
In the ADA chart, they usually come in after the lower baby canines (17–23 months) and before the upper baby second molars (25–33 months), and they are among the last to fall out, along with the upper baby second molars and the upper baby canines.
Tooth K is replaced by tooth 20, the lower left second premolar, which usually comes in at 11–12 years (ADA). Under the gum, tooth K starts to harden in the 4th month of pregnancy and is fully formed by about 36 months (AAPD).
When the premolar under tooth K is missing
The lower second premolar (tooth 20), which should replace this tooth, is the adult tooth most often missing from birth (wisdom teeth aside). When it is missing, this baby molar may stay for years.
A sinking baby molar. If this tooth fuses to the bone (ankylosis) and seems to sink below its neighbors, it can be a sign the premolar under it is missing.
Keeping it as an adult. Without a premolar, this baby molar may be kept or removed. Because it is wider than a premolar, keeping it can affect the bite, and keeping a sunken one for years can leave a dip in the bone that makes a later implant harder.
The same tooth on the right side, tooth T, has its own section on the extra room it holds for adult teeth.
Where is tooth K?
Tooth K is in your child's lower jaw on their left side. It is the fifth tooth from the middle: start at the gap between the two bottom front teeth and count 5 teeth toward the back on their left side.
"Left" and "right" always mean the child's own sides, so when you face your child, tooth K is on your right.
In front of it (toward the middle) is tooth L, the lower left baby first molar. Behind it, the adult first molar (tooth 19) usually comes in at 6–7 years without replacing any baby tooth.
Its mirror twin on the right side is tooth T. The tooth in the same spot in the upper jaw is tooth J, the upper left baby second molar.
Dental charts show the patient's right side on the left, so tooth K sits at the far right end of the bottom row. The letters then drop to the lower jaw: lower left teeth run from K (the back molar) to O (the front tooth).
What does the lower baby second molar do?
The lower baby second molar is the biggest baby tooth in the lower jaw. It looks like a smaller version of the adult lower first molar that comes in right behind it. Baby teeth matter even though they fall out: children need them to chew, speak and smile, and they help the adult teeth come in correctly.
How many roots does tooth K have?
2 roots, front and back. Baby molar roots are thin and spread wide apart. The premolar that will replace the baby molar forms in the space between them.
Common problems with lower baby second molars
- Cavities. The AAPD advises repairing decayed baby teeth when possible, rebuilding each tooth to its original shape so it keeps its place until the adult tooth is ready.
If a baby tooth is knocked loose or out in a fall, or your child has swelling, fever or pain, call your dentist. Never put a knocked-out baby tooth back in (Knocked-Out Tooth Rescue explains why). Swelling that makes it hard to breathe or swallow is an emergency: call 911. These are typical problems for this kind of tooth, not a diagnosis.
Tooth K in FDI and Palmer notation
| System | Tooth K is written | What it means |
|---|---|---|
| Universal (ADA, used in the US) | K | Baby teeth are lettered A–T around the mouth, starting at the upper right back molar. |
| FDI / ISO 3950 (international) | 75 | First digit 7 = lower left quarter (baby teeth use 5–8); second digit 5 = fifth baby tooth from the middle. Said "seven-five". |
| Palmer | ┌E (often typed LLE) | The ┌ bracket marks the lower left quarter; E = fifth baby tooth from the middle. |
Palmer stops at E. Palmer letters only run from A to E in each quarter, so a "K" can only be a Universal letter.
FDI baby teeth start with 5 to 8. The first digit 7 marks the lower left baby quarter, so FDI 75 can't be mixed up with an adult tooth.
Letters and numbers don't line up. K is letter 11 of the alphabet, but baby tooth K is replaced by adult tooth 20, not tooth 11.
All baby teeth in the lower left quarter
From the front of the mouth to the back:
| Universal | FDI | Palmer | Tooth |
|---|---|---|---|
| O | 71 | ┌A (LLA) | Baby central incisor |
| N | 72 | ┌B (LLB) | Baby lateral incisor |
| M | 73 | ┌C (LLC) | Baby canine |
| L | 74 | ┌D (LLD) | Baby first molar |
| K | 75 | ┌E (LLE) | Baby second molar (this tooth) |
The Tooth Number Chart & Converter converts any tooth between the three systems.
Tools for tooth K
- Teething Calculator: which baby teeth are due at your baby's age.
- Baby Teeth Tracker: log each tooth in and out.
- Kids Fluoride Calculator: how much toothpaste to use; the ADA says a rice-grain smear until age 3, then a pea-size amount.
- Tooth Fairy Calculator: what families give for a lost tooth.
- Dental Treatment Plan Decoder: plain English for treatment-plan shorthand like "#30 MOD comp".
Frequently asked questions
What tooth is letter K?
When does tooth K come in?
When does tooth K fall out?
What if there is no adult tooth under tooth K?
Sources
- American Dental Association (MouthHealthy). Permanent Tooth Development (eruption chart). 2012. Accessed October 26, 2026.
- American Dental Association (MouthHealthy). Primary Tooth Development (eruption chart). 2012. Accessed October 26, 2026.
- American Academy of Pediatric Dentistry. Dental Growth and Development (chronology after Logan & Kronfeld 1933, adapted 2003). Accessed October 26, 2026.
- American Academy of Pediatric Dentistry. Management of the Developing Dentition and Occlusion in Pediatric Dentistry (Reference Manual of Pediatric Dentistry). latest revision 2024. Accessed October 26, 2026.
- World Health Organization. Oral health surveys: basic methods, 5th edition (tooth notation after ISO 3950). 2013. Accessed October 26, 2026.
- Nelson SJ. Elsevier. Wheeler's Dental Anatomy, Physiology and Occlusion, 11th edition. 2019.
- American Dental Association. ADA Dental Claim Form (tooth designation systems). Accessed October 26, 2026.
- Dentalcare.com (continuing education course CE500). An Overview of Dental Anatomy: Permanent Dentition. Accessed October 26, 2026.
- American Dental Association (MouthHealthy). Tooth Decay with Baby Bottles. Accessed October 26, 2026.
Last reviewed by the Dental Health Time editorial team, against the sources above. How we review.