Your hair’s thickness comes down to two things: how wide each individual strand is and how many strands you have growing at once. Both are largely determined by your genetics, though hormones, nutrition, and even your stage of life play a role. Understanding which type of “thick” you’re dealing with helps explain why your hair behaves the way it does.
Strand Thickness vs. Hair Density
When people say “thick hair,” they usually mean one of two different things, and the distinction matters. The first is strand diameter, which is how wide each individual hair is. Terminal hairs (the pigmented, fully developed hairs on your scalp) are classified into three categories: thin strands measure below 30 micrometers across, medium strands fall between 30 and 50 micrometers, and thick strands exceed 50 micrometers. For reference, 50 micrometers is about half the width of a sheet of paper.
The second factor is density, meaning how many hairs are packed into each square centimeter of your scalp. A person with fine individual strands can still have extremely thick-feeling hair if they have high density, and someone with coarse, wide strands might feel like they have less volume if their follicle count is low. Most people who describe their hair as “thick” have some combination of both wider strands and higher density.
Genetics Are the Biggest Factor
The single largest influence on your hair’s thickness is your DNA. Several genes control follicle size, and one of the most studied is the EDAR gene. Variations in this gene are strongly associated with thicker, straighter hair strands, particularly in East Asian populations. But EDAR is just one of many genes involved. Hair thickness is a polygenic trait, meaning dozens of genetic variants each contribute a small effect that adds up to the hair you see in the mirror.
Your genetic background also influences total follicle count. Blondes tend to have the highest density at roughly 150,000 follicles, followed by brown-haired individuals at around 110,000, black-haired at 100,000, and redheads at about 90,000. These numbers are set before birth and don’t change over your lifetime. You can’t grow new follicles, only lose existing ones. So if you’ve always had thick hair, your parents gave you both wide follicles and a lot of them.
How Ethnicity Shapes Hair Diameter
A large study measuring hair from young adults across 24 ethnic groups on five continents found that Asian hair consistently had the thickest diameter and the fastest growth rate. African-textured hair tends to have a smaller cross-section but a more elliptical shape, which creates curl and the visual appearance of volume. European hair generally falls somewhere in between. These are broad patterns with enormous individual variation, but they help explain why people from certain backgrounds are more likely to deal with high-volume, coarse hair.
Hormones Can Change Your Hair Over Time
If your hair wasn’t always this thick, hormones are a likely explanation. During puberty, rising androgen levels transform many of the tiny, nearly invisible vellus hairs on your body into larger, darker terminal hairs. This same process can increase the diameter of scalp hair strands. The final size a hair reaches depends on how long the follicle stays in its active growth phase, called anagen, and androgens directly influence that duration.
Pregnancy is another common trigger. Elevated estrogen keeps a higher percentage of hair follicles in the growth phase for longer, which means less daily shedding and noticeably fuller hair. This effect reverses within three to six months after delivery, when hormone levels drop and all those retained hairs enter the shedding phase at once. Thyroid imbalances can also shift hair texture in either direction, making hair feel coarser or finer depending on whether the thyroid is overactive or underactive.
Conditions like polycystic ovary syndrome (PCOS) can cause excess androgen production, which sometimes thickens body and facial hair while paradoxically thinning scalp hair in certain areas. If your hair texture has changed suddenly or you’re noticing thicker, darker hair in new places, a hormonal shift is worth investigating.
Nutrition and Hair Structure
Hair strands are composed almost entirely of a protein called keratin, so your diet directly affects what your follicles can build. Protein intake is the most critical nutritional factor. Your body needs the amino acid cysteine as a key building block for keratin, and it gets this from protein-rich foods like eggs, fish, poultry, and legumes. Eating keratin supplements directly doesn’t help because the body can’t absorb intact keratin proteins. Instead, you need to consume the raw amino acids your follicles use to construct it.
B vitamins also play a supporting role. Research on dietary supplements containing the amino acid L-cystine combined with B1 and B5 has shown improvements in hair quality and tensile strength. This doesn’t mean supplements will make already-thick hair thicker, but it does mean that nutritional deficiencies can prevent your hair from reaching the fullness your genetics would otherwise allow. Iron, zinc, and adequate calories all matter for the same reason: hair follicles are metabolically demanding tissue, and they’re among the first things your body deprioritizes when nutrients are scarce.
Managing Thick Hair Day to Day
Thick hair has different care needs than fine hair. The American Academy of Dermatology recommends that people with thick, curly, or textured hair shampoo less frequently, roughly once every two to three weeks or as needed, since thick hair tends to be drier and doesn’t accumulate oil as quickly at the ends. When you do condition, apply it along the entire length of the strand rather than just the tips.
Detangling is best done in the shower while conditioner is still in, using a wide-tooth comb rather than a brush. Wet hair is more fragile and elastic, so pulling a fine-bristled brush through thick, tangled wet hair causes breakage. Heat styling is another concern. Thick hair often takes longer to blow-dry or straighten, which means more cumulative heat exposure. Using a low or medium heat setting with a heat-protectant product reduces damage over time. Choosing products specifically labeled for thick or coarse hair types will also make a noticeable difference, as these formulas tend to be more moisturizing and better at smoothing the cuticle layer that gives thick strands their rougher texture.