Dental calculus is roughly 70 to 90 percent mineral by dry weight, with the remaining 10 to 30 percent made up of organic material: bacteria, proteins, and cellular debris. It forms when dental plaque, the soft sticky film on your teeth, absorbs calcium and phosphate ions and hardens into a mineralized deposit that can’t be brushed or flossed away.
The Mineral Component
The bulk of dental calculus is calcium phosphate in various crystal forms. Calcium makes up about 39 percent of the total composition, and phosphorus accounts for roughly 19 percent. Together they form several distinct crystal types that coexist within the same deposit: hydroxyapatite (the same mineral found in tooth enamel and bone), whitlockite, brushite, and octacalcium phosphate. Smaller amounts of calcium carbonate (about 3 percent), magnesium phosphate, and trace metals like zinc, iron, sodium, copper, and fluoride round out the mineral profile.
Which crystal forms dominate depends partly on where the calculus sits and how old it is. Younger deposits tend to contain more brushite and octacalcium phosphate, while older, more mature calculus shifts toward hydroxyapatite and whitlockite as the crystals reorganize over time.
The Organic Component
The non-mineral portion of calculus is about 85 percent cells and 15 percent extracellular matrix, the structural scaffolding that holds those cells in place. Cell density is remarkably high: an estimated 200 million cells per milligram. Most of those cells are bacteria, but the mix also includes at least one species of archaea (a separate domain of single-celled organisms) and several species of yeast, including Candida albicans.
Beyond microorganisms, calculus traps trace amounts of material from your mouth and diet. Salivary proteins, plant DNA from food, milk proteins, starch granules, textile fibers, and even smoke particles have all been identified within calculus deposits. Human DNA gets passively incorporated too, primarily from saliva and other oral secretions. This is why ancient dental calculus has become a valuable tool for archaeologists: it preserves a snapshot of what people ate, breathed, and carried in their mouths thousands of years ago.
How Plaque Turns Into Calculus
Calculus begins as ordinary dental plaque, which is a biofilm of bacteria living in a sticky matrix on tooth surfaces. Mineralization starts when calcium and phosphate ions, dissolved in saliva, become concentrated enough to crystallize within this biofilm. Two things push the process forward. First, bacteria in plaque produce an enzyme called alkaline phosphatase, which releases extra phosphate ions locally. Second, pH rises at the plaque surface, and that shift toward a more alkaline environment makes calcium phosphate less soluble, causing it to drop out of solution and form solid crystals.
The initial deposits are amorphous, meaning the calcium phosphate doesn’t yet have a defined crystal structure. Over time, these unstable clusters reorganize through several intermediate steps into the more stable crystal forms like hydroxyapatite. The process can begin within days of plaque accumulating on a tooth, and once a small amount of calculus forms, its rough surface makes it easier for more plaque to stick and mineralize on top, creating a self-reinforcing cycle.
Above the Gumline vs. Below It
Dental calculus forms in two distinct locations, and the mineral source differs for each. Supragingival calculus, the visible yellowish or white buildup above the gumline, draws its minerals from saliva. It tends to accumulate most heavily near the openings of salivary glands: the inside surfaces of lower front teeth and the outer surfaces of upper molars, where saliva flow is highest.
Subgingival calculus forms below the gumline, in the narrow pocket between the tooth and gum tissue. Its minerals come not from saliva but from gingival crevicular fluid, a serum-based liquid that seeps from the blood supply in the gums. Because of this different mineral source, subgingival calculus tends to be darker (often brown or black), denser, and more firmly attached to the root surface. It’s also harder for a dentist or hygienist to detect and remove, since it’s hidden beneath the gum tissue.
What Lives Inside the Mineral Matrix
As calculus matures, the bacterial community inside it shifts. Early biofilm tends to contain aerobic bacteria, the kind that thrive in oxygen-rich environments near the tooth surface. But as layers build up and the interior becomes sealed off from air, anaerobic species take over. Mature calculus is enriched with these late-colonizer bacteria that flourish in low-oxygen conditions.
The specific mix of microbes also varies by location within the mouth. Interproximal spaces, the tight gaps between teeth, tend to harbor higher levels of acid-tolerant anaerobes. Lower-mass deposits have a different bacterial profile than thick, heavy accumulations. This microbial diversity is one reason calculus contributes to gum disease: it provides a protected, mineralized shelter where harmful bacteria can thrive right against the tooth and gum tissue, beyond the reach of your toothbrush.