Fingernails are made almost entirely of keratin, a tough structural protein that also forms hair and the outer layer of skin. The nail plate itself is about 80–90% hard keratin, with the remainder being water (roughly 7–18%), a small amount of fat (under 5%), and trace minerals. Despite what many people believe, calcium plays almost no role in making nails hard.
Keratin: The Main Building Block
Keratin comes in two broad categories in humans: soft keratin, found in the outer layer of your skin, and hard keratin, the type found in hair and nails. Your nail plate is 80–90% hard keratin, with the remaining 10–20% being soft keratin. Hard keratin gets its toughness from a high concentration of sulfur-containing amino acids, particularly cystine. These sulfur atoms form strong chemical bridges called disulfide bonds that lock the protein chains together into a rigid, resilient structure.
At the elemental level, keratin is roughly 45% carbon, 15% nitrogen, 14% oxygen, 7% hydrogen, and 3% sulfur. The remaining 8–10% of the nail plate consists of inorganic components like phosphates and carbonates. Two types of hard keratin, one acidic and one basic, pair together to build the layered structure of the nail plate. This pairing is what gives nails their characteristic combination of hardness and slight flexibility.
Water and Fat Content
Healthy fingernails contain about 12% water on average, though the normal range spans roughly 7–18%. This water content matters more than you might expect. Nails that lose too much moisture become brittle and prone to cracking, while nails exposed to excess water (from prolonged soaking, for example) become soft and peel more easily.
Fat makes up less than 5% of the nail plate by weight. These lipids are arranged in a layered structure running parallel to the nail surface, where they help regulate how much water moves in and out of the nail. This is why nail oils and moisturizers can help maintain nail flexibility, since they support the lipid barrier that keeps hydration balanced.
The Calcium Myth
One of the most persistent misconceptions about nails is that calcium makes them hard. Calcium actually makes up less than 0.03% of the nail plate by weight, a tiny fraction compared to the 20–25% calcium concentration in bone. Studies have found no evidence that calcium supplements improve nail strength or that calcium deficiency causes brittle nails. The real source of nail hardness is the dense network of disulfide bonds within the keratin protein structure, not mineral content.
How Nails Grow
Nails are produced by the nail matrix, a crescent-shaped tissue mostly hidden beneath the skin at the base of your nail. Living skin cells in the matrix rapidly divide and undergo a transformation called cornification: they flatten out, lose their internal structures, fill up with keratin, and essentially die. The result is a tough, translucent cell called an onychocyte. As new onychocytes form, they push older ones forward, creating the visible nail plate that slides slowly along the nail bed toward your fingertip.
Fingernails grow at an average rate of about 3.5 millimeters per month, which works out to roughly a tenth of a millimeter per day. That’s more than twice as fast as toenails, which average around 1.6 millimeters per month. Growth speed varies by finger (the middle finger tends to grow fastest), by age (nails slow down as you get older), and by season (growth picks up slightly in warmer months).
Parts of the Nail
The visible part you trim and paint is just the nail plate, but the full nail unit has several working parts:
- Nail plate: The hard, slightly curved surface with subtle vertical ridges. The white tip that extends past your fingertip is called the free edge.
- Nail matrix: The growth factory hidden under the skin at the base of your nail. Damage to the matrix can permanently alter how the nail grows.
- Nail bed: The pink skin directly under the nail plate, rich with tiny blood vessels that nourish the fingertip. It has matching ridges that interlock with the nail plate and hold it in place.
- Cuticle: The thick band of skin along the base of the nail that seals the gap between the nail plate and the surrounding skin, acting as a barrier against bacteria and fungi.
What Nail Appearance Can Tell You
Because nails grow slowly and are built from proteins your body assembles over weeks, they can reflect changes in your overall health. Horizontal ridges or grooves running across the nail (called Beau’s lines) can appear after a period of illness, severe stress, or nutritional deficiency, marking a point when nail growth temporarily slowed or paused. Spoon-shaped nails that curve inward sometimes indicate iron deficiency. Yellowing or thickening often points to fungal infection.
Vertical ridges running from base to tip, on the other hand, are usually harmless and become more prominent with age. White spots are almost always caused by minor trauma to the matrix, not by any vitamin deficiency. Since the nail plate is made of dead cells by the time you can see it, changes in appearance reflect something that happened weeks earlier, during the time those cells were being formed in the matrix.