Final answer:
Hair density in humans is measured per square centimeter, which provides a precise measurement given the small diameter of human hair, which averages about 20 micrometers.
Step-by-step explanation:
Hair density is often discussed in the subject of Biology, particularly when considering human anatomy or the characteristics of mammals. When measuring hair density, which ranges from thick to thin, it's imperative to use a scale that is precise enough to give meaningful results. For human hair, density is typically measured per square centimeter.
The average diameter of a human hair is about 20 micrometers (μm). Given this small diameter, a larger measurement unit like a square foot or even a square inch would not provide the necessary precision. The square centimeter allows for a more accurate representation of hair density on the human scalp or body of other mammals.