A single human hair can be thinner than a silk thread or nearly twice as thick as a sheet of paper.

The diameter of human hair is measured in micrometers, also known as microns. Research from the Journal of Cosmetic Science indicates that the average human hair ranges from 17 to 180 micrometers in diameter. For context, a micrometer is one-millionth of a meter. This wide range explains why hair can feel drastically different from person to person.Genetic factors and ethnicity play a significant role in these measurements. Studies have shown that Asian hair is typically the thickest, often averaging around 80 to 120 micrometers. In contrast, Caucasian hair is usually finer, averaging between 50 and 90 micrometers. These differences are rooted in the structure of the hair follicle and the amount of keratin packed into the hair shaft.The internal structure of the hair also dictates its thickness. Every strand consists of three layers: the medulla, the cortex, and the cuticle. Coarser hairs often possess a prominent medulla, which is the innermost core, while very fine hairs may lack this layer entirely. The thickness of the cortex, which contains the pigment and provides strength, accounts for the majority of the hair's diameter.Environmental factors and age can also alter hair thickness over time. As people age, hair follicles can shrink, leading to a reduction in the diameter of the hair produced. This process, known as miniaturization, is a key component of age-related hair thinning. Understanding these microscopic measurements helps scientists develop specific hair care products tailored to different hair textures and strengths.