Human nails are made almost entirely of a tough protein called keratin, the same material found in hair and the outer layer of skin. The nail plate itself is a layered structure of flattened, dead cells packed with keratin fibers and held rigid by sulfur-based chemical bonds. Small amounts of water, lipids, and trace minerals round out the composition, but protein dominates.
Keratin: The Main Building Block
The specific type of keratin in nails is alpha-keratin, a fibrous protein built from long chains of amino acids twisted into coil-like shapes called alpha-helices. These coils bundle together into tiny fibers called microfibrils, which sit within a surrounding matrix of keratin-associated proteins. Think of it like rebar embedded in concrete: the microfibrils provide internal reinforcement while the matrix fills in and binds everything together.
What makes nails so much harder than skin (which also contains keratin) comes down to sulfur. Nails are rich in an amino acid called cysteine, which contains sulfur atoms. When two cysteine molecules sit close together, their sulfur atoms link up to form what’s called a disulfide bond. These bonds act like tiny cross-bridges locking the protein chains to one another, creating a dense, rigid network. The more disulfide bonds, the harder the material. Nails have far more of these cross-links than skin does, which is why you can’t dent your nail the way you can press into your forearm.
Hydrogen bonds also contribute to nail stiffness, though they behave differently from disulfide bonds. Water can break hydrogen bonds but leaves disulfide bonds intact. This is why your nails become softer and more flexible after a long bath but harden again once they dry out.
Three Layers, One Plate
A nail isn’t a single uniform slab. It’s a laminated structure made of three tightly bound layers: the dorsal (top), intermediate (middle), and ventral (bottom). Each layer has a slightly different arrangement of keratin fibers. In the intermediate layer, the fibers run sideways across the nail, which reinforces the plate’s overall structural integrity. The dorsal layer is the densest and acts as the main barrier against chemicals and moisture penetrating the nail. The ventral layer connects to the nail bed underneath.
This layered design is part of what makes nails both strong and slightly flexible. If you’ve ever noticed a nail peeling in thin sheets at the tip, you’re seeing these layers separating from one another.
Water Content and Nail Flexibility
Despite feeling dry and solid, nails contain water. Estimates vary, but some experts put normal nail water content around 18%, with brittleness potentially developing below 16%. However, a study published in the Journal of the American Academy of Dermatology found the picture is more complicated. Researchers measured water content in both brittle and normal nails and found averages of about 12% in both groups, with no significant difference between them. So while hydration plays a role in how flexible a nail feels moment to moment, chronically brittle nails aren’t necessarily drier than healthy ones. Other factors like lipid content and disulfide bond integrity likely matter just as much.
The Role of Lipids
A small but important fraction of the nail plate is made up of lipids (fats), particularly ceramides. These waxy molecules sit between the flattened nail cells, forming a network of hydrogen bonds with surrounding water molecules. Ceramides help the nail retain moisture and maintain its barrier function, preventing excessive water loss from the nail surface.
When ceramides are disrupted, nails suffer. Acetone-based nail polish remover, for example, penetrates the nail plate and breaks up the ceramide structure, stripping moisture from between the layers. This is why frequent polish remover use can leave nails peeling and brittle. The most abundant type of ceramide in fingernails is the dihydrosphingosine type, and research has shown that both sphingosine and phytosphingosine ceramides decrease after exposure to nail polish remover.
Trace Minerals
Nails also contain small amounts of minerals, including calcium, zinc, iron, and magnesium. These minerals are present in trace quantities and are sometimes used in research as biomarkers for overall mineral status in the body. Despite popular belief, calcium is not the primary source of nail hardness. That job belongs to keratin’s disulfide bonds. The calcium in nails is incidental, not structural.
How Nails Are Made
The nail plate is produced by a patch of tissue called the nail matrix, which sits just beneath the skin behind your cuticle. Living skin cells in the matrix divide rapidly and undergo a transformation process called cornification. During cornification, the cells flatten dramatically, lose their internal structures (nucleus, organelles), and fill up with densely packed keratin. By the time these cells reach the visible nail plate, they’re essentially dead, compacted protein discs called onychocytes, stacked and fused into a hard sheet.
This production line never stops. Fingernails grow about 3.47 millimeters per month, while toenails are slower at roughly 1.62 millimeters per month. A completely lost fingernail takes about three to six months to regrow, while a toenail can take over a year. Growth rate varies with age, circulation, and season, with nails tending to grow faster in summer and on your dominant hand.
Why This Composition Matters
Understanding what nails are made of explains most of the nail problems people encounter. Brittle, splitting nails often trace back to disrupted lipids or damaged protein bonds, not a calcium deficiency. Nails that soften after water exposure are losing hydrogen bonds temporarily. Peeling at the tips reflects separation of the nail’s three structural layers. And slow growth usually connects to circulation or nutrition rather than any single missing mineral.
Your nails are, at their core, a remarkably engineered protein composite. Keratin provides the foundation, sulfur cross-links provide the hardness, ceramides seal the gaps, and water keeps the whole structure from becoming too rigid. Remove or alter any one of those components and the nail’s performance changes noticeably.