A human hair is a thin strand made mostly of protein, and a typical hair from the head is about 0.1 millimeters (100 micrometers) wide. That is its width straight across the strand, not its length. A micrometer is a thousandth of a millimeter.

Size

The U.S. National Nanotechnology Initiative gives the width of a human hair as 80,000–100,000 nanometers, which is 80–100 micrometers. A nanometer is a thousandth of a micrometer. The width of 100 micrometers shown here is at the top of that range.

Hair varies a great deal from person to person. A 2014 study of scalp hair puts the width of adult human hair at about 20–180 micrometers, so the thickest hairs are about nine times as wide as the finest. Scientists measure hair width under microscopes and with laser instruments.

Size comparisons

About 10 hairs laid side by side span 1 millimeter, the smallest mark on a metric ruler.
A thou, one-thousandth of an inch, is exactly 25.4 micrometers, so a typical hair is 3–4 thou wide.
A sheet of office paper is about 0.1 millimeters thick, about the same as the width of a hair.
About 11–13 red blood cells, each about 7.5 micrometers across, would fit side by side across a hair.
A human egg cell, or ovum, is about 0.12 millimeters across, a little wider than a typical hair.

What a hair is made of

Most of a hair is keratin, a tough protein. Cut across, a hair shows three layers. At the center is the medulla, a loosely packed region. Around it is the cortex, which holds most of the hair's mass.

The cortex is built from spindle-shaped cells about 1–6 micrometers across and 50–100 micrometers long, lined up along the hair. Outside it is the cuticle, a protective layer of flat, dead cells that overlap like scales. Each cuticle cell is only about 0.5 micrometers thick. Scalp hair generally grows to about 90 centimeters (35 inches) long, about 9,000 times its width.

Why hairs differ in width

Studies comparing people from around the world find large differences in hair width between population groups. On average, hair from people of Asian and African ancestry is thicker than hair from people of European ancestry, and Asian hair is the thickest.

A hair's width also depends on which way it is measured, because most hairs are not perfectly round. In cross section, a hair is usually an oval that is flatter in some people than others. Straight hair tends to be the most nearly round, and tightly curled hair the flattest. Even a single long hair can change along its length: one study found hairs becoming rounder and finer toward the root.

Sources

Just How Small Is "Nano"?. National Nanotechnology Coordination Office.
The structure of people's hair. Yang et al., PeerJ, 2014.
Quantifying whole human hair scalp fibres of varying curl: A micro-computed tomographic study. van den Berg et al., Journal of Microscopy, 2024.
NIST Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically. National Institute of Standards and Technology.
18.3 Erythrocytes. In Anatomy and Physiology, 2nd edition. OpenStax, 2022.
Oocyte diameter predicts the maturation rate of human immature oocytes collected ex vivo. Pors et al., Journal of Assisted Reproduction and Genetics, 2022.

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