A dental formula is a shorthand way of recording how many of each tooth type sit in one half of the mouth, then multiplying by two to get the total tooth count. The formula always lists four tooth types in the same order: incisors (I), canines (C), premolars (P), and molars (M). The upper jaw sits on top and the lower jaw on the bottom, separated by a horizontal line, just like a fraction.
How the Formula Is Structured
Each dental formula represents only one side of the mouth. Because mammal jaws are symmetrical, whatever appears on the left side mirrors the right. The upper row shows the teeth in the upper jaw (maxilla), and the lower row shows the teeth in the lower jaw (mandible). Written out, it looks like this:
I C P M
Upper: 2 1 2 3
Lower: 2 1 2 3
That example is the adult human dental formula. It tells you that on one side of the upper jaw, there are 2 incisors, 1 canine, 2 premolars, and 3 molars. The lower jaw has the same count. You may also see it written in a condensed form with periods: 2.1.2.3 over 2.1.2.3.
Calculating the Total Tooth Count
Getting from the formula to a total number of teeth takes two quick steps.
Step 1: Add one row. Take the numbers for one side of the upper jaw and add them together. For an adult human, that’s 2 + 1 + 2 + 3 = 8 teeth on one side of the upper jaw. Do the same for the lower jaw: 2 + 1 + 2 + 3 = 8.
Step 2: Multiply by two. Since the formula only covers one side of the mouth, double the upper total and double the lower total, then add them together. For the human adult: upper is 8 × 2 = 16, lower is 8 × 2 = 16, and 16 + 16 = 32 total permanent teeth.
You can also shortcut this by adding both rows first (8 + 8 = 16 for one side), then multiplying by 2 to get 32. Either method gives you the same answer.
Human Baby Teeth Use a Different Formula
Children don’t have the same dental formula as adults. The deciduous (baby) set has no premolars at all, and only two molars per quadrant instead of three. The formula is:
Upper: 2 1 0 2
Lower: 2 1 0 2
Running the same calculation: each side of the upper jaw has 2 + 1 + 0 + 2 = 5 teeth. The lower jaw matches with 5. One full side of the mouth has 10 teeth, and 10 × 2 = 20 total baby teeth. Later, the baby molars are replaced by premolars, and three permanent molars erupt behind them to complete the adult set of 32.
Applying the Formula to Other Species
The real power of dental formulas shows up in comparative anatomy, where the upper and lower jaw counts often differ. Dogs, for instance, have 42 permanent teeth: 12 incisors, 4 canines, 16 premolars, and 10 molars. Their formula is:
Upper: 3 1 4 2
Lower: 3 1 4 3
One side of the upper jaw: 3 + 1 + 4 + 2 = 10. One side of the lower jaw: 3 + 1 + 4 + 3 = 11. Total for one side: 21. Multiply by 2: 42 teeth. Notice the lower jaw has one more molar per side than the upper jaw, which is why the two rows aren’t identical.
An even more dramatic example is the Virginia opossum, which has one of the highest tooth counts of any land mammal. Its formula is 5.1.3.4 over 4.1.3.4, yielding 50 total teeth. The upper side has 13 teeth (26 across both sides) while the lower has 12 (24 across both sides).
Referencing Individual Teeth
Dental formulas also let you pinpoint a specific tooth using subscripts and superscripts. The convention is straightforward: a superscript number indicates an upper tooth, and a subscript number indicates a lower tooth. So M² refers to the second upper molar, while I₁ refers to the first lower incisor. In deciduous teeth, lowercase letters are used instead. The first baby incisor is written as di₁, and the second baby molar is dm₂.
Why Dental Formulas Vary Between Species
Tooth number and shape are closely tied to diet. Over millions of years, natural selection has tuned each species’ dentition to match what it eats. Fruit-eating primates tend to have relatively larger incisors for dehusking fruit and seeds, while leaf-eating primates have smaller incisors but molars with taller cusps and sharper shearing crests for grinding tough, low-energy leaves. Insect-eating primates have smaller molars with sharp cusps designed to puncture exoskeletons.
These differences make dental formulas one of the most reliable tools for identifying species, both living and extinct. Paleontologists use them to reconstruct evolutionary relationships from fossil jaws. The modern human formula of 2.1.2.3, along with the loss of the large interlocking canines seen in earlier primates, first appeared in Miocene-era apes roughly 5 to 22 million years ago. Because tooth enamel preserves exceptionally well in the fossil record, dental formulas have become a cornerstone of tracing how lineages branched and adapted over time.