Hair and nails grow from the same basic process: specialized cells rapidly divide, fill with a tough protein called keratin, and then harden as they push outward. Scalp hair grows about half an inch per month, fingernails about 3.5 millimeters per month, and toenails roughly half that speed. The biology driving this growth depends on blood supply, hormones, and a steady intake of specific nutrients.
How Hair Grows in Cycles
Unlike nails, which grow continuously, hair follows a four-phase cycle. The active growth phase, called anagen, lasts two to eight years for scalp hair. During this time, cells in the hair follicle divide rapidly, stacking new keratin-filled cells on top of one another and pushing the hair strand upward. The length of your anagen phase is largely genetic, which is why some people can grow hair to their waist while others plateau at shoulder length.
After anagen, the follicle enters a brief regression phase lasting about two weeks, where cell division slows and the lower portion of the follicle shrinks. Then comes a resting phase of two to three months, during which the hair strand stays in place but is no longer actively growing. Finally, the old strand sheds and a new one begins forming underneath. At any given time, roughly 85 to 90 percent of your scalp hairs are in the active growth phase, which is why you don’t notice the constant cycling.
How Nails Grow Continuously
Nail growth originates in the nail matrix, a pocket of tissue tucked beneath the skin at the base of each nail. The matrix constantly produces new cells that flatten, harden with keratin, and slide forward as the visible nail plate. The pale half-moon shape you can see at the base of some nails, called the lunula, is the visible edge of this matrix. The upper portion of the matrix creates the top layer of the nail, while the lower portion forms the underside, producing a layered structure that gives nails their strength.
Fingernails grow about twice as fast as toenails, averaging 3.47 millimeters per month compared to 1.62 millimeters. Your dominant hand typically grows nails slightly faster, likely because of increased blood flow from more frequent use. Nails also grow faster during the daytime and in summer months, when circulation to the extremities increases with warmer temperatures.
Keratin: The Protein Behind Both
The structural material in both hair and nails is alpha-keratin, a fibrous protein with a helical, screw-like shape. This is the same type of keratin found in the outer layer of your skin. The difference in hardness between a strand of hair and a fingernail comes down to how tightly the keratin fibers are packed and how much sulfur-rich bonding holds them together. Nails have a denser arrangement of these bonds, making them rigid rather than flexible. Your body assembles keratin from amino acids, which is why protein intake has a direct relationship to how well both hair and nails grow.
Nutrients That Fuel Growth
Building keratin requires a steady supply of raw materials. Protein is the most fundamental, since keratin itself is a protein. People on very low-protein diets often notice brittle nails and thinning hair before other symptoms appear. Beyond protein, several micronutrients play specific roles in the process.
Iron carries oxygen to hair follicles and nail matrix cells, and low iron stores are one of the most common nutritional causes of hair shedding. Research on women with hair loss found that optimal hair growth occurred when serum ferritin (the body’s stored iron) reached about 70 nanograms per milliliter, well above the 20 nanograms per milliliter that many labs consider the minimum “normal” cutoff. This means your iron levels can technically be in the normal range and still be too low for your hair to thrive.
Zinc supports collagen production and helps maintain the oil glands around hair follicles. Biotin, a B vitamin, is widely marketed for hair and nails, though true deficiency is rare in people eating a varied diet. Vitamin B12 also appears to matter: studies have linked levels between 300 and 1,000 nanograms per liter with better hair growth outcomes.
How Hormones Speed Up or Slow Down Growth
Hormones are among the most powerful regulators of hair growth, which is why hair changes so noticeably during puberty, pregnancy, and menopause. Estrogen extends the active growth phase of hair follicles by binding to receptors on the follicle itself. This is why many women experience noticeably thicker hair during pregnancy, when estrogen levels are high, followed by a wave of shedding a few months after delivery when those levels drop.
Progesterone plays a protective role by reducing the conversion of testosterone into a more potent form called DHT. DHT shrinks hair follicles over time, particularly at the temples and crown. This is the central mechanism behind pattern hair loss in both men and women. Thyroid hormones also affect growth rate: an underactive thyroid slows cell turnover throughout the body, often causing hair thinning and nails that grow slowly, become brittle, or develop ridges.
Blood Flow and Seasonal Changes
Both hair and nails depend on blood supply to deliver oxygen and nutrients to their growth centers. Anything that increases circulation to those areas tends to modestly speed up growth. This is why nails grow faster in summer, when blood vessels in your hands and feet dilate in the warmth. It also helps explain why the fingernails on your dominant hand grow slightly faster and why toenails, which have less blood flow than fingernails, grow at roughly half the rate.
Scalp massage has been studied for its effects on hair thickness, with some evidence suggesting that regular manipulation increases blood flow to follicles. Exercise has a similar indirect benefit by improving overall circulation. On the other hand, smoking constricts blood vessels and has been linked to premature graying and hair loss.
What Doesn’t Actually Help
Trimming your hair does not make it grow faster. Growth starts at the follicle, deep in the scalp, and cutting the ends has zero effect on what happens there. What trimming does accomplish is removing split ends before they travel up the shaft, which helps you retain more length over time. So regular trims can make it look like your hair is growing faster, even though the actual rate hasn’t changed.
Gelatin supplements, once a popular home remedy for stronger nails, have never been shown to work in controlled studies. And while biotin supplements are heavily marketed, clinical evidence of benefit is limited to people with an actual biotin deficiency, which is uncommon. For most people, the nutrients that matter most for hair and nail growth, like iron, zinc, protein, and B vitamins, are better obtained through food than supplements, unless a blood test reveals a specific shortage.
Why Growth Slows With Age
Hair and nail growth both decelerate as you get older. Hair follicles gradually spend less time in the active growth phase and more time resting, which means individual strands don’t grow as long before shedding. The follicles themselves can also shrink, producing thinner strands. Nail growth slows by roughly 0.5 percent per year after age 25, which is why older adults often notice their nails seem to take longer to need trimming. Reduced blood flow, lower hormone levels, and decreased cell turnover all contribute to this slowdown.